Properly defer entrance animations until after the stack is reloaded.

- Change the enter animation animated event to a normal event (no
  animations were being choreographed on the event animation trigger
  anyways)
- Remove the code in RecentsActivity, which poorly tried to schedule
  the animation after the first layout.  Instead, properly defer the
  start animation until both the first layout _after stack reload_
  and enter animation callback happens (regardless of which one comes
  first).

Bug: 38391395
Test: Launch Chrome, dock, and ensure there are tasks visible
Change-Id: I4c2823fe25adbd3c200a0f786c478155541ccf26