Don't draw taskbar background during gestures

In this case, the ScrimView for BackgroundAppState will handle drawing behind the whole screen, and prevents taskbar from drawing the rounded corners on top of TaskViews.

Added a hack to defer resuming last task by a frame to ensure taskbar is drawing first.

Test: Quick switch to an app, ensure it isn't covered by the taskbar's rounded corner
Bug: 196257194
Change-Id: Ibc81d80bc3bf1d908e6912b179cbcafbdcc56c0f