[PiP2] Cleanup Bitmap and buffer after setting up

- This is a counterpart of ag/31545168
- PiP2 owns a copy of PipAppIconOverlay due to config-at-end, i.e. the
  actual scale of the overlay is different from PiP1
- Changed also in PiP2 PipTransition to reference the PiP2 static method
- Deprecated also the unused class variable

Flag: com.android.wm.shell.enable_pip2
Bug: 393584443
Test: manually, make sure the Bitmap is recycled and \
      no function regression
Change-Id: I3afb61d93dfe5ea7a880c45521667d7e1149f983