Implement stashing in PiP2

Implement stashing fling gesture and animation in PiP2
via transitions as per go/pip2-transitions.

Bug: 352119868
Flag: com.android.wm.shell.enable_pip2_implementation
Test: fling to stash PiP and tap to unstash
Change-Id: I4ed0882de013301322ff9f57bb7943a89c2dbbfc