Fix issue with black flash when swiping up

- Ensure we update the background paint whenever the thumbnail is
  bound to the task view (we still draw with the background paint when the
  thumbnail has not yet loaded)
- Ensure that the dim is applied to the background paint even before
  the thumbnail has loaded

Bug: 122612839
Test: Kill launcher, launch overview
Change-Id: Ia09ff49e4641b6a02a37cec3f94f74c2b0b8fd24