"Merge" activity-only transitions into ongoing recents

Since activity-only transitions don't impact recents
state, we can just apply them directly without interrupting
the current recents animation.

Bug: 269691226
Test: launch an app which launches an activity (into itself)
      after a delay (eg. notification permissions). Enter
      recents before that happens.
Change-Id: I356bcc93b975f9979f8ff6cda3c190c2e9ad3a3f