Fix 4689527: Fix positioning bug with RecentHorizontalScrollView, minor tweaks.

This fixes a bug where RecentsHorizontalScrollView wasn't scrolling to the most
recent app.

Contains some minor tweaks to the layout so that it doesn't overlap the notification
bad as well as starting alpha fades while draging items sooner.

Change-Id: I94df309673f316c55b2f44aff60fd3aee746b6de