[DO NOT MERGE] Make SetWallpaperCallback independent of Activity

Use a ViewModel to keep track of the wallpaper set status
so that if the Activity is restarted because of a
configuration change before the wallpaper set callback
returns, the new Activity still finished once it's applied.

Bug: 194080642
Test: manual
Change-Id: I3d4933caa73dcebf3711512dedfef3b024958630
Merged-In: I3d4933caa73dcebf3711512dedfef3b024958630
5 files changed