Write Icon to proto

This change adds RemoteViewsSerializers hidden class which contains
some of the serialization code for RemoteViews. This CL adds code for
writing/reading Icons from proto.

This is to facilitate serializing RemoteViews actions that contain
Icons to proto.

Test: RemoteViewsSerializersTest#writeIconToProto
Bug: 345800969
Flag: android.appwidget.flags.remote_views_proto
Change-Id: Ia52b781e1c4ee12cec6c6255703df189f9c02b5d