Stop signing Mono.Android.dll.

From https://learn.microsoft.com/en-us/dotnet/standard/assembly/strong-named:

For .NET Core and .NET 5+, strong-named assemblies do not provide material benefits. The runtime never validates the strong-name signature, nor does it use the strong-name for assembly binding.

For .NET 9 Preview 1, we should stop signing Mono.Android.dll, and see if anything breaks.