Clean up status bar, system bar, navigation bar management.

The status bar and navigation bar are two completely separate
elements, with their own semantics.  The system bar now classifies
itself as a navigation bar, since that is really how it behaves.

This required rewriting the HDMI resizing code, so that it is
all done by PhoneWindowManager since that is what is responsible
for the size of the navigation bar (and thus now system bar).  This
actually gets rid of a fair amount of code, and means we can also
do the same thing for a pure navigation bar.

Likewise the system bar now has the navigation bar ability to be
hidden when requested by system UI flags.  To get the behavior
we want on Xoom, we only allow the nav bar to be hidden when it
will help provide a better aspect ratio for showing widescreen
videos.

Finally the nav/system bar now animates when hidden and shown.

Change-Id: Ie927154b68376a0b61802f99171ff56b8da92e7a