[One .NET] specify AssemblyMetadataAttribute for IsTrimmable (#828)

Context: dotnet/android#5638
Context: dotnet/android#5879

Going forward in .NET 6, we shouldn't use the `%(IsTrimmable)`
metadata anymore, but instead use the following C# attribute in
each assembly for which linking/"trimming" is supported:

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

Similar changes on the iOS side here:

  * dotnet/macios@289053b

Add `AssemblyMetadataAttribute` to `Java.Interop.dll`.

Author: jonathanpeppers

src/Java.Interop/Properties/AssemblyInfo.cs

@@ -13,6 +13,7 @@
 [assembly: AssemblyProduct ("")]
 [assembly: AssemblyTrademark ("Microsoft Corporation")]
 [assembly: AssemblyVersion ("0.1.0.0")]
+[assembly: AssemblyMetadata ("IsTrimmable", "True")]
 
 [assembly: InternalsVisibleTo (
 	"Java.Interop.GenericMarshaler, PublicKey=" +