Write Spanned strings to proto

Adds code to serialize Spanned strings to proto. All the spans that
implement ParcelableSpan are supported.

This is to facilitate serializing RemoteViews actions that contain Spanned
strings.

Test: RemoteViewsSerializersTest
Bug: 345800969
Flag: android.appwidget.flags.remote_views_proto
Change-Id: I4497f5ae10140f58771a7d8f915d994af9eecc0f
8 files changed