The fullest of fullscreen modes.

View.setSystemUiVisibility() now properly accepts a
bitfield, including:

  * SYSTEM_UI_FLAG_LOW_PROFILE: "lights out mode"
    (previously known, erroneously, as STATUS_BAR_HIDDEN)

  * SYSTEM_UI_FLAG_HIDE_NAVIGATION: for when you need every
    single pixel on a device that also has a navigation bar

These flags are painstakingly aggregated across the entire
view hierarchy and carefully delivered to the status bar
service, which in turn gently passes them along to the bar
implementation.

To really get access to the whole screen, you need to use
HIDE_NAVIGATION in conjunction with FLAG_FULLSCREEN and
FLAG_LAYOUT_IN_SCREEN. See development/samples/Overscan for
an example of how to do this.

Change-Id: I5fbfe009d9ceebbbf71db73f14a7008ea7c1d4da