Make with the clicky. (Status bar buttons now play FX.)

Now sending accessibility events and playing click noises:
 - Home
 - Back
 - Notification icons (if you tap; tap-and-hold/scrub is
   silent)
 - the clock area

I also added touch slop to the main nav buttons (should have
been there all along).

Bug: 3422943
Change-Id: Ib5ee1d3a7c79e60e05377715092cde3f7678ddcf