[Xamarin.Android.Build.Tasks] emit exported="true" for MauiLauncher a…

…ctivities

Context: https://aster.cloud/2021/02/23/lets-be-explicit-about-our-intent-filters/
Fixes: dotnet#6196

> An important change is coming to Android 12 that improves both app
> and platform security. This change affects all apps that target
> Android 12.
>
> Activities, services, and broadcast receivers with declared
> intent-filters now must explicitly declare whether they should be
> exported or not.

For example:

    <activity android:icon="@mipmap/icon" android:label="HelloWorld" android:name="example.MainActivity">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>

Should become:

    <activity android:icon="@mipmap/icon" android:label="HelloWorld" android:name="example.MainActivity">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" android:exported="true" />
        <category android:name="android.intent.category.LAUNCHER" android:exported="true" />
      </intent-filter>
    </activity>

Update our `AndroidManifest.xml` generation & related code so that
when `MainLauncher=true`, we also automatically set `exported=true`.

I updated a test to verify the contents of the generated
`AndroidManifest.xml`.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs

@@ -141,12 +141,32 @@ public void CheckElementReOrdering ([Values (true, false)] bool useAapt2)
 				var manifestFile = Path.Combine (Root, builder.ProjectDirectory, proj.IntermediateOutputPath, "android", "AndroidManifest.xml");
 				XDocument doc = XDocument.Load (manifestFile);
 				var ns = doc.Root.GetNamespaceOfPrefix ("android");
-				var manifest = doc.Element ("manifest");
-				Assert.IsNotNull (manifest, "manifest element should not be null.");
-				var app = manifest.Element ("application");
-				Assert.IsNotNull (app, "application element should not be null.");
+				var manifest = getElement (doc, "manifest");
+				var app = getElement (manifest, "application");
 				Assert.AreEqual (0, app.ElementsAfterSelf ().Count (),
 					"There should be no elements after the application element");
+				var activity = getElement (app, "activity");
+				var intent_filter = getElement (activity, "intent-filter");
+				var action = getElement (intent_filter, "action");
+				assertAttribute (action, ns + "name", "android.intent.action.MAIN");
+				assertAttribute (action, ns + "exported", "true");
+				var category = getElement (intent_filter, "category");
+				assertAttribute (category, ns + "name", "android.intent.category.LAUNCHER");
+				assertAttribute (category, ns + "exported", "true");
+			}
+
+			static XElement getElement (XContainer parent, XName name)
+			{
+				var e = parent.Element (name);
+				Assert.IsNotNull (e, $"{name} element should not be null.");
+				return e;
+			}
+
+			static void assertAttribute (XElement parent, XName name, string expected)
+			{
+				var a = parent.Attribute (name);
+				Assert.IsNotNull (a, $"{name} attribute should not be null.");
+				Assert.AreEqual (expected, a.Value, $"{name} attribute value did not match.");
 			}
 		}
 

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -797,11 +797,12 @@ void AddLauncherIntentElements (XElement activity)
 			if (activity.Elements ("intent-filter").Any (f => IsMainLauncher (f)))
 				return;
 
+			var exported = new XAttribute (androidNs + "exported", "true");
 			var filter = new XElement ("intent-filter");
 			activity.AddFirst (filter);
 			foreach (KeyValuePair<string, string> e in LauncherIntentElements) {
 				if (!filter.Elements (e.Key).Any (x => ((string) x.Attribute (attName)) == e.Value))
-					filter.Add (new XElement (e.Key, new XAttribute (attName, e.Value)));
+					filter.Add (new XElement (e.Key, new XAttribute (attName, e.Value), exported));
 			}
 		}