[PiP2] Prevent double enter PiP

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

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

Bug: 420614827
Flag: com.android.wm.shell.enable_pip2
Test: manually enter content PiP in multiple quick successions.
Change-Id: Ie023dfaa3e6c7f81522babb37c51d345f172389d