Fixes: dotnet#1157

The remaining trimmer warnings are around usage of System.Reflection
such as:

* Create an `Invoker` type from an original type via
  `Type.Assembly.GetType()`

* Create a `Type` based on a `ParameterInfo.ParameterType` and/or
  create instances

* Use `Type.MakeGenericType()`

* Use `Type.MakeArrayType()`

The trimming warnings themselves all indicate that these types "might
be trimmed", in that nothing explicitly preserves them. However, we
have a plethora of custom trimmer steps that work to preserve these
types involved in Java interop, such as:

* https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveApplications.cs
* https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveExportedTypes.cs
* https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveJavaExceptions.cs
* https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveJavaInterfaces.cs
* https://github.com/xamarin/xamarin-android/blob/main/src/Microsoft.Android.Sdk.ILLink/PreserveRegistrations.cs

One day, in a perfect world, we could remove these trimmer steps and
completely rely on the trimmer's warnings & attributing mechanisms.
That day doesn't have to be today -- as our goal is begin surfacing
trimmer warnings to users in their own code and dependencies.

With these warnings ignored, we can fully enable analyzers with:

    <IsTrimmable>true</IsTrimmable>
    <EnableAotAnalyzer>true</EnableAotAnalyzer>

We can then remove:

    [assembly: AssemblyMetadata ("IsTrimmable", "True")]

As the MSBuild property does this for us, in addition to enabling
analyzers.

I don't think we should enable `$(IsAotCompatible)` quite yet, as
`Java.Interop.dll` likely *won't work* yet on NativeAOT, and this
places metadata that says an assembly is supported. We should just use
the `$(EnableAotAnalyzer)` analyzers for now, so we don't introduce
new issues.