Subtract the navigation bar from DisplayMetrics.

Its region is now treated like the system bar: inaccessible
to applications and therefore not worth reporting as part of
the display.

(Note that using setSystemUiVisibility you can gain
temporary access to the navigation bar region, unlike the
sempiternal system bar.)

The navigation bar is now considerably less in control of
its own behavior (the window manager assumes it will be a
certain size and in a certain position in landscape and
portrait). This change also fixes the navbar so that it
becomes GONE instead of merely INVISIBLE (allowing
underlying windows to expand in size accordingly).

Bug: 5052456 // the feature
Bug: 5067627 // notification shade falling behind the navbar
Bug: 4959805 // fix third-party apps relying on DisplayMetrics
Change-Id: I60257fc5c858e0edcaba8cfc1f8d58dd99a68258