Empty motion bounds when PiP bounds changed

When PipTouchHandler#movePip is triggered, the motion bounds will be set
and never gets reset. The littering motion bounds therefore blocks any
double-tap resize.

Fix this by empty the motion bounds whenever the resize transition is
finished.

Cleanup also the pip1 references in pip2 PipTouchHandler

Flag: com.android.wm.shell.enable_pip2
Bug: 404338655
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/c4UUsVjSRd2t7XMPLwIe90
Test: Drag and pinch-resize PiP, see Video
Change-Id: I92854665eefd5d4c05cc6a055cf11ec52df7a5a3