Fix FAB positioning for incoming calls.

We aren't setting the screen width in onResume, which means that
repositions would fail for incoming calls, or resumes. Now, set the
screen width when resuming the app.

I also cleaned up some of the method naming to be more precise; there
shouldn't be a change in behavior resulting from it.

The OnGlobalLayout listener wasn't triggering if the observer wasn't
alive, so address this by checking for a new observer.

Bug: 17673299
Change-Id: I1d454f4a5d1561bc73c12a6c74edbc075e1f1a18