[PiP2] Generalize legacy-enter filter

We need to make isLegacyEnter check more
generic and accurte to allow for legacy-enter cases
that can have other changes collected into the TRANSIT_PIP
transition as well, such as trampoline activity launch case
reported in the bug below.

Also adding a wtf log for cases where enter PiP was scheduled
one way or another but PipTransition.startAnimation is about to
be skipped as a playing handler.

Bug: 403637164
Flag: com.android.wm.shell.enable_pip2
Test: manually repro the steps in b/403637164#comment8
Test: legacy-enter PiP from a Whatsapp call (with a task filling TF)
Change-Id: I16139501d5126192cf51a79e21522a48df8cd644