Finish the primary container while secondary exited

When finishing a placeholder activity, the secondary container was
clean up, and the primary activity was also finished. However, the
primary container didn't remove and therefore was expanded to
fullscreen before the activity within completely being removed.

Which resulted in TRANSIT_OLD_TASK_FRAGMENT_CHANGE transition
after commit 118428b6 merged.

Bug: 198228078
Test: finishing placeholder activity and back to launcher
Change-Id: I3d55f0be8c7f6ab3970450d7b90dd95f3cfdd463