Cache and reuses LauncherAppWidgetHostView when launcher resumes

Currently by design when launcher enters the background, it stops
listening to updates in widgets. This eventually causes the dilemma
for launcher when it resumes, before the update can be returned from
the system process via IPC, launcher could do one of the following
to fill the gap:
1. show a deferred widget view -- a placeholder that renders the shape
   of the widget -- to let the user know widget is being reloaded.
2. show whichever widget view that was previously displayed to the
   user that may now contain stale content.

There is a descrepancy here since in some edge cases we are showing the
former while in most other cases we are showing the later. This CL added
a short-term fix to address the descrepancy and favors the later where
possible.

Bug: 218067434
Test: manual
Change-Id: I6cd2cd704186267227e2ec47f2581843fd526fa0