[Xamarin.Android.Build.Tasks] Debug-mode linker improvements

~~ Mono.Cecil ~~

During incremental Debug builds, the `<LinkAssemblies/>` MSBuild task
already has logic to skip linking on assemblies that are unchanged.
There is a `LinkOnlyNewerThan` property that enables this behavior for
the `_LinkAssembliesNoShrink` MSBuild target.

However, we are still *loading* these assemblies into memory with
Mono.Cecil.

I was able to prevent assemblies from being loaded into memory by:

- Moving the `skiplist` logic to the beginning of the MSBuild task.
- Only `Load ()` the assemblies that we are linking
- Preserve the assembly search directories to the linker, so it *can*
  load these assemblies if needed on-demand.

This should not modify the behavior for Release builds (builds running
`_LinkAssembliesShrink`).

~~ Results ~~

Incremental builds are a bit faster, due to not loading as many
assemblies:

  Before:
  392 ms  LinkAssemblies                             1 calls
  After:
  157 ms  LinkAssemblies                             1 calls

Saved ~250ms in a "Hello World" Xamarin.Forms app, after modifying XAML.

~~ StripEmbeddedLibraries ~~

I have noticed the support libraries from NuGet are fairly large. In
total I have seen them around 8MB in size! Much of their size is not
in fact managed code, but the Android resources, jar files, etc.
contained within them.

For example, if you look at the size of
`__AndroidLibraryProjects__.zip` embedded in
Xamarin.Android.Support.v7.MediaRouter 27.0.2.1:

    592534 __AndroidLibraryProjects__.zip

~0.5MB in a single assembly!

So I did an experiment to run the `StripEmbeddedLibraries` linker step
during a debug build.

    Before:
    9.66 MB total
      22064 FormsViewGroup.dll
     101376 HelloForms.Android.dll
       6144 HelloForms.dll
      35416 Xamarin.Android.Arch.Core.Common.dll
      55904 Xamarin.Android.Arch.Lifecycle.Common.dll
      31840 Xamarin.Android.Arch.Lifecycle.Runtime.dll
      77432 Xamarin.Android.Support.Animated.Vector.Drawable.dll
     151648 Xamarin.Android.Support.Annotations.dll
    1699408 Xamarin.Android.Support.Compat.dll
     545880 Xamarin.Android.Support.Core.UI.dll
     228952 Xamarin.Android.Support.Core.Utils.dll
     744528 Xamarin.Android.Support.Design.dll
     409688 Xamarin.Android.Support.Fragment.dll
     733280 Xamarin.Android.Support.Media.Compat.dll
     322648 Xamarin.Android.Support.Transition.dll
      41544 Xamarin.Android.Support.v4.dll
    1995872 Xamarin.Android.Support.v7.AppCompat.dll
      60504 Xamarin.Android.Support.v7.CardView.dll
     780896 Xamarin.Android.Support.v7.MediaRouter.dll
      68184 Xamarin.Android.Support.v7.Palette.dll
     851552 Xamarin.Android.Support.v7.RecyclerView.dll
      61024 Xamarin.Android.Support.Vector.Drawable.dll
     673336 Xamarin.Forms.Core.dll
     334416 Xamarin.Forms.Platform.Android.dll
      16960 Xamarin.Forms.Platform.dll
      85560 Xamarin.Forms.Xaml.dll

    After:
    5.60 MB total
      12800 FormsViewGroup.dll
     101376 HelloForms.Android.dll
       6144 HelloForms.dll
      16896 Xamarin.Android.Arch.Core.Common.dll
      28160 Xamarin.Android.Arch.Lifecycle.Common.dll
      13824 Xamarin.Android.Arch.Lifecycle.Runtime.dll
      32256 Xamarin.Android.Support.Animated.Vector.Drawable.dll
     118784 Xamarin.Android.Support.Annotations.dll
    1142272 Xamarin.Android.Support.Compat.dll
     301568 Xamarin.Android.Support.Core.UI.dll
     123904 Xamarin.Android.Support.Core.Utils.dll
     350208 Xamarin.Android.Support.Design.dll
     231936 Xamarin.Android.Support.Fragment.dll
     411136 Xamarin.Android.Support.Media.Compat.dll
     137216 Xamarin.Android.Support.Transition.dll
      28672 Xamarin.Android.Support.v4.dll
     947200 Xamarin.Android.Support.v7.AppCompat.dll
      28672 Xamarin.Android.Support.v7.CardView.dll
     181248 Xamarin.Android.Support.v7.MediaRouter.dll
      35328 Xamarin.Android.Support.v7.Palette.dll
     497152 Xamarin.Android.Support.v7.RecyclerView.dll
      20480 Xamarin.Android.Support.Vector.Drawable.dll
     673336 Xamarin.Forms.Core.dll
     334416 Xamarin.Forms.Platform.Android.dll
      16960 Xamarin.Forms.Platform.dll
      85560 Xamarin.Forms.Xaml.dll

This equates to:
- 4.0 MB less assemblies shipped to the device during deployment!
- 4.0 MB less assemblies loaded at startup!

Changes to make this happen:

- For this to work properly, I had to change `StripEmbeddedLibraries`
  to operate on `Skip` assemblies. If the assembly is going to get
  stripped, then its action will change to `Save`. This now means this
  pass has to run *last*, so a comment seemed ideal in the list of
  linker steps.

~~ Results ~~

The same "Hello World" Xamarin.Forms project had a minor improvement
to the initial deployment:

    Before:
    2730 ms  InstallPackageAssemblies                   1 calls
    After:
    2549 ms  InstallPackageAssemblies                   1 calls

Saved ~200ms during the first deployment.

And if you review the size of the assemblies copied during
`InstallPackageAssemblies`:

    Before:
    100% ... 10332kb of 10332kb copied
    After:
    100% ... 6173kb of 6173kb copied

TODO: I need to measure startup time impact.

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/Linker.cs

@@ -61,6 +61,7 @@ static Pipeline CreatePipeline (LinkerOptions options)
 
 			if (options.LinkNone) {
 				pipeline.AppendStep (new FixAbstractMethodsStep ());
+				pipeline.AppendStep (new StripEmbeddedLibraries ());
 				pipeline.AppendStep (new OutputStep ());
 				return pipeline;
 			}
@@ -113,6 +114,7 @@ static Pipeline CreatePipeline (LinkerOptions options)
 			// monodroid tuner steps
 			if (!string.IsNullOrWhiteSpace (options.ProguardConfiguration))
 				pipeline.AppendStep (new GenerateProguardConfiguration (options.ProguardConfiguration));
+			//NOTE: StripEmbeddedLibraries should run last since Skip assemblies might change to Save
 			pipeline.AppendStep (new StripEmbeddedLibraries ());
 			// end monodroid specific
 			pipeline.AppendStep (new RegenerateGuidStep ());

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/StripEmbeddedLibraries.cs

@@ -14,7 +14,7 @@ protected override void ProcessAssembly (AssemblyDefinition assembly)
 			if (!Annotations.HasAction (assembly))
 				return;
 			var action = Annotations.GetAction (assembly);
-			if (action == AssemblyAction.Skip || action == AssemblyAction.Delete)
+			if (action == AssemblyAction.Delete)
 				return;
 
 			var fileName = assembly.Name.Name + ".dll";
@@ -33,7 +33,7 @@ protected override void ProcessAssembly (AssemblyDefinition assembly)
 					}
 				}
 			}
-			if (assembly_modified && action == AssemblyAction.Copy) {
+			if (assembly_modified && (action == AssemblyAction.Copy || action == AssemblyAction.Skip)) {
 				Annotations.SetAction (assembly, AssemblyAction.Save);
 			}
 		}

src/Xamarin.Android.Build.Tasks/Tasks/LinkAssemblies.cs

@@ -79,12 +79,34 @@ public override bool Execute ()
 
 		bool Execute (DirectoryAssemblyResolver res)
 		{
+			var skiplist = new List<string> ();
+
+			if (string.Compare (UseSharedRuntime, "true", true) == 0)
+				skiplist.AddRange (Profile.SharedRuntimeAssemblies.Where (a => a.EndsWith (".dll")).Select (a => Path.GetFileNameWithoutExtension (a)));
+			if (!string.IsNullOrWhiteSpace (LinkOnlyNewerThan) && File.Exists (LinkOnlyNewerThan)) {
+				var newerThan = File.GetLastWriteTime (LinkOnlyNewerThan);
+				var skipOldOnes = ResolvedAssemblies.Where (a => File.GetLastWriteTime (a.ItemSpec) < newerThan);
+				foreach (var old in skipOldOnes)
+					Log.LogMessage (MBF.MessageImportance.Low, "  Skip linking unchanged file: " + old.ItemSpec);
+				skiplist = skipOldOnes.Select (a => Path.GetFileNameWithoutExtension (a.ItemSpec)).Concat (skiplist).ToList ();
+			}
+
+			// Add LinkSkip options
+			if (!string.IsNullOrWhiteSpace (LinkSkip))
+				foreach (var assembly in LinkSkip.Split (',', ';'))
+					skiplist.Add (assembly);
+
 			// Put every assembly we'll need in the resolver
 			foreach (var assembly in ResolvedAssemblies) {
-				res.Load (Path.GetFullPath (assembly.ItemSpec));
+				res.SearchDirectories.Add (Path.GetDirectoryName (assembly.ItemSpec));
+				if (!skiplist.Contains (Path.GetFileNameWithoutExtension (assembly.ItemSpec)))
+					res.Load (Path.GetFullPath (assembly.ItemSpec));
 			}
 
 			var resolver = new AssemblyResolver (res.ToResolverCache ());
+			foreach (var assembly in res.SearchDirectories) {
+				resolver.AddSearchDirectory (assembly);
+			}
 
 			// Set up for linking
 			var options = new LinkerOptions ();
@@ -101,24 +123,6 @@ bool Execute (DirectoryAssemblyResolver res)
 			options.HttpClientHandlerType = HttpClientHandlerType;
 			options.TlsProvider = TlsProvider;
 			options.PreserveJniMarshalMethods = PreserveJniMarshalMethods;
-
-			var skiplist = new List<string> ();
-
-			if (string.Compare (UseSharedRuntime, "true", true) == 0)
-				skiplist.AddRange (Profile.SharedRuntimeAssemblies.Where (a => a.EndsWith (".dll")).Select (a => Path.GetFileNameWithoutExtension (a)));
-			if (!string.IsNullOrWhiteSpace (LinkOnlyNewerThan) && File.Exists (LinkOnlyNewerThan)) {
-				var newerThan = File.GetLastWriteTime (LinkOnlyNewerThan);
-				var skipOldOnes = ResolvedAssemblies.Where (a => File.GetLastWriteTime (a.ItemSpec) < newerThan);
-				foreach (var old in skipOldOnes)
-					Log.LogMessage (MBF.MessageImportance.Low, "  Skip linking unchanged file: " + old.ItemSpec);
-				skiplist = skipOldOnes.Select (a => Path.GetFileNameWithoutExtension (a.ItemSpec)).Concat (skiplist).ToList ();
-			}
-
-			// Add LinkSkip options
-			if (!string.IsNullOrWhiteSpace (LinkSkip))
-				foreach (var assembly in LinkSkip.Split (',', ';'))
-					skiplist.Add (assembly);
-
 			options.SkippedAssemblies = skiplist;
 
 			if (EnableProguard)