Add RemoteViews.createPreviewFromProto/writePreviewToProto

This CL adds support for writing RemoteViews to proto. Caches, actions,
and support for serializing certain types will be added in followup
CLs.

Test: RemoteViewsProtoTest
Bug: 308041327
Flag: android.appwidget.flags.remote_views_proto
Change-Id: If0de92c417d0dfd9ead5894fd2874d41b77dc39e