Remove LockscreenRecentsActivity

- This activity would create another task (no longer in the same
  stack since we don't have activity type stacks) which was causing
  the camera to be occluded sometimes, which then cancels the recents
  animation
- Tweaking the animation slightly to match the movement of the closing
  the activity back to the lockscreen

Bug: 156514461
Test: Launch camera on the lockscreen and swipe up to dismiss (repeatedly)
Change-Id: I8aaac22767288d7fbf183683276c50acaaf831d2