Avoid empty TaskFragment removal affecting playing transition

A transition will apply the finish transaction from
buildFinishTransaction when it finishes. But if there is a parallel
transition runs and finishes before the previous transition, the
layer assignment may have conflict. Even if the transition doesn't
contain changes (shell will abort it), its finish transaction is
still applied by shell's Transitions#onFinish.

E.g. when recents transition (allow parallel) is running, an app
with embedded activities deletes its empty TaskFragment when its
activity is removed by recents (swipe up task view).

By not collect invisible TF and not return WCT effect for empty TF
removal, applyTaskFragmentTransactionLocked can abort the empty
transition in WM core if the effect is none and the transition does
not contain participants.

Fix: 381466040
Flag: EXEMPT bugfix
Test: TaskFragmentOrganizerControllerTest# \
      testApplyTransaction_enforceHierarchyChange_deleteTaskFragment
Change-Id: Ibe86b36ea675330d63b90dda30332d0d4220489d