Check AR attached upon enter PiP collect

Since collection could be delayed if some
other sync is ongoing for a different transition
we should check if a PiP candidate activity is still
attached before collecting anything or making changes in
the TRANSIT_PIP transition and bail early if needed.

Bug: 438555132
Flag: EXEMPT bugfix
Test: N/A
Change-Id: I7ce72c2b802f2508ace2a45e074116961c317649