Fix bug 5037642 - Refine back button behavior for action bar modes.

Action modes for the action bar used to aggressively hijack the back
button before the view hierarchy got a chance to respond to it. Loosen
this. New ordering is: view hierarchy => window feature-level modes
(action bar modes e.g. CAB/search) => Activity/window callback
behavior.

Change-Id: Iac1b22997713be968a94f77f9fa6cebaf6f923f0