Tuning performance for back to home transition.

Basically revert 8b1aed02, shell transiiton will capture task snapshot
once onTransitionReady signal send to shell, or capture in
RecentsTransitionHandler#getSnapshotsForPausingTasks, so the capture
action shouldn't related to display anymore, therefore doesn't need to
trigger capture task snapshot during prepare transition.
TaskStackChangeListeners#onTaskStackChanged shall ask
RecentsModel#onTaskStackChangedBackground to load task thumbnail.

Bug: 307389404
Test: open app -> back to -> enter overview -> launch app. Verify on
perfetto that capture snapshot won't happen while prepare transition,
and capture snapshot should only happen once.

Change-Id: Ic54b95570a7e1fe832c7faa84f17b3d0de95ee97