[One .NET] use AssemblyMetadataAttribute for IsTrimmable

Fixes: #5638

Going forward in .NET 6, we shouldn't use the `%(IsTrimmable)`
metadata anymore, but use the following C# attribute in each assembly
instead:

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

Similar changes on the iOS side here:

dotnet/macios@289053b

Likely, I will need to add `AssemblyMetadataAttribute` for
`Java.Interop.dll`, before this is fully working.

Author: jonathanpeppers

build-tools/create-packs/Microsoft.Android.Sdk.proj

@@ -111,7 +111,6 @@ WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and
         RuntimePackNamePatterns="Microsoft.Android.Runtime.**RID**"
         RuntimePackRuntimeIdentifiers="@(_AndroidNETAppRuntimePackRids, '%3B')"
         Profile="Android"
-        IsTrimmable="true"
     />
   </ItemGroup>
 </Project>

external/Java.Interop

@@ -16,6 +16,7 @@ using System.Runtime.CompilerServices;
 // FIXME: Probably need to add Copyright 2009-2011 Novell Inc.
 // [assembly: AssemblyCopyright ("Copyright 2011-2014 Xamarin Inc.")]
 [assembly: AssemblyCompany ("Microsoft Corporation")]
+[assembly: AssemblyMetadata ("IsTrimmable", "True")]
 
 [assembly: System.Runtime.CompilerServices.TypeForwardedToAttribute(typeof(System.Drawing.Color))]
 [assembly: System.Runtime.CompilerServices.TypeForwardedToAttribute(typeof(System.Drawing.ColorConverter))]