[generator] Override methods should match base method's deprecated info.

[jpobst]

There are several warnings when building Mono.Android.dll related to Obsolete methods:

warning CS0672: Member 'MediaRouteActionProvider.OnCreateActionView()' overrides obsolete member 'ActionProvider.OnCreateActionView()'. Add the Obsolete attribute to 'MediaRouteActionProvider.OnCreateActionView()'

Technically, MediaRouteActionProvider.onCreateActionView is marked as deprecated in the docs, but not in the compiled android.jar:

public class MediaRouteActionProvider extends ActionProvider {
  public View onCreateActionView() {
    throw new RuntimeException("Stub!");
  }
}

Regardless, any method that overrides a deprecated method should itself be marked as deprecated.

Another case specific to Mono.Android.dll is ContextWrapper.setWallpaper

This method is marked as deprecated-since = 23, but the base method is marked as deprecated-since = 16. This causes us to generate:

public class Context {
  [Obsolete]
  public virtual void SetWallpaper () { ... }
}

public class ContextWrapper : Context {
  [ObsoletedOSPlatform ("android23.0")]
  public override void SetWallpaper () { ... }
}

This causes the same warning. For these cases, we set the override method's deprecated-since to match the base method.