RemoteViews: make bitmaps immutable

Because mutability is propagated across Parcel and Binder, any mutable
Bitmaps passed to RemoteViews will end up mutable on the other side
even though those Bitmaps can't be modified from the RemoteViews
recipient.

Make all Bitmaps passed to RemoteViews immutable in order to ensure
that they can be sent without additional copies instead of the one
ashmem object.

Test: TH, RemoteViews work for widgets
Bug: 227920378

Change-Id: I477e7132f9ff78333c1eda56b9d0fc6ac520abd0