Implement app icon overlay when entering PiP

This is done by
- draw a bitmap with the theme background and app icon at its center
- reparent the hardware buffer of that bitmap to the task leash
- transform the bitmap leash to be unscaled and centered during the
  animation

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/cyuEoL0G1HOS8hjfRmlyPt
Bug: 265998256
Test: turn on the flag and enter PiP, see video
Change-Id: Ia7354d83e9b5701f1358b44b0db7bb32d48ccd2a