Properly prevent All Apps relayouts by avoiding scrollToPosition

Calling scrollToPosition on RecyclerView internally calls
requestLayout() (to cacluate where to scroll and then go there).
Therefore, we should avoid calling that whenever possible, especially
during transitions. In particular, we can optimize scrollToTop() to not
scrollToPosition() if we are already at the top.

This makes some other workarounds unnecessary, namely setting All Apps
to GONE during system gestures.

Test: Open an app, swipe up, ensure AllAppsRecyclerView doesn't get
onLayout().  If we had scrolled to an app first, we get one layout
in prepareRecentsUi(), but not during the transition.

Bug: 140308849
Change-Id: I62ee341bf5893c121cfc013cc6542559f79d2a42