Continue deferring removal of activities in a transition

Otherwise its possible for transitions to become ready early
and trigger bad behavior (such as promoting up to displayarea).

Basically, this fixes a bug where isExitAnimationRunning was
returning false for windows in shell-transitions

Bug: 239887306
Test: full wipe, complete set-up wizard until homescreen
      or atest DigitsKeyListenerTest
Change-Id: Iba0454a27e65c976de00adb7ae94b1e0bfe9affc