Removing legacy default thumbnail logic.

- With the move to snapshots, we no longer keep a local cache of the
  thumbnails and tasks which have no thumbnails will be handled
  in the snapshot from the system (if it uses FLAG_SECURE) or in
  Recents directly through the task description background color.

  In the case of this bug, the default thumbnail (1x1 white bitmap) was
  incorrectly being applied because the system could not take a snapshot
  of the app due to DRM.

Bug: 62447895
Test: Open any app with DRM, play video, tap Recents

Change-Id: I02e87cf513eb9750b39d54d085b0c19645b7ed52