Add tabletop mode support to PiP2

Respect and respond to Tabletop / Flex mode change
- When Foldable in tabletop mode, defaults PiP position to the top-half
- When change Foldable to tabletop mode, moves PiP to top-half if
  applicable

Internally, it's implemented as KCA (Keep-Clear-Area).

Flag: com.android.wm.shell.enable_pip2
Bug: 411161949
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/ejoA7C7s6Cwxfzt2LQQAPr
Test: Enter PiP while in Flex mode
Test: Enter Flex mode while PiP is present
Change-Id: I98a4b11bc29172a4217d691958447556753c7c82