[PIP2] Attach PiP menu to leash.

Recall: http://recall/clips/a4248f21-10d6-422c-837c-30117540bac4
Winscope: https://screenshot.googleplex.com/6yTWd3Q5HZKZhsb

Bug: 322548939
Test: Launch PiP and verify menu is present and app actions are usable.
Flag: com.android.wm.shell.enable_pip2
Change-Id: I9d21f576b7764bb5d697573f93be0260e2367327