On-screen navigation bar (separate from the status bar).

In Honeycomb we introduced navigation controls in the status
bar, for xlarge devices without physical buttons. What about
phones? The status bar is pretty cramped already, and
besides, it's at the top of the display most of the time,
not at the bottom where your thumb is likely to be.

Enter the navigation bar. It's a new window type that
appears atop almost everything (including the keyguard); the
window manager subtracts its rectangle from the default
visible rectangle of other windows (including the status bar
and notification shade).

However, it behaves (on phones) like the status bar in that
applications that request fullscreen windows can get access
to those pixels. Well, almost; they need cooperation from
the navigation bar implementation to make the navbar
disappear, just like the status bar.

The current SystemUI implementation of the navigation bar on
phones is still rough, but it has the basics:

 + back, home, and menu keys (NB: we're showing menu all the
   time right now because checking the api level of the
   package owning the top window is currently a poor
   indicator of whether the app requires the menu key)
 + it tries to stick to the same physical end of the device,
   regardless of device orientation (on a phone, this is
   the strip of land closest to the microphone)

Change-Id: Ic613a3351220af0bbfbdef63e1d99cbefd5ed1c2