[PiP2] Prevent double enter PiP

Prevent PiP entry being scheduled if there
is one scheduled already and it hasn't started playing yet.

Make sure we restore the internal state if a scheduled-enter-pip
transition gets aborted, so that we can re-enter PiP in a follow-up
session.

Also refactored the content-pip filter checks with
a new PipUtils utility.

Bug: 420614827
Flag: com.android.wm.shell.enable_pip2
Test: atest PinnedStackTests
Test: manually enter content PiP in multiple quick successions.
Change-Id: Idaeaf032120df21cee7dab44fd0d4ca2d5dbe8b9