Add new feature to let apps layout over status bar / system bar.

The main change is a few new flags you can supply to
View.setSystemUiVisibility().  One is a new visibility mode,
SYSTEM_UI_FLAG_FULLSCREEN, which is basically the same as
the global FLAG_FULLSCREEN option for windows, but driven as
part of the system UI state.

There are also three new flags for telling the framework that you
would like to have your application's UI ignore screen
decorations -- SYSTEM_UI_FLAG_LAYOUT_NO_NAVIGATION for going
behind the navigation bar and SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
for ignoring full screen decorations (that is the status bar).

In combination with this you can use SYSTEM_UI_FLAG_LAYOUT_STABLE
to have the framework report consistent insets to your application.

When using NO_NAVIGATION, when the user taps the screen we now
also automatically clear ONLY_CONTENT, so that we atomically show
both UI elements.  This should make it easy for apps like video
players that want to move between fully full-screen and regular
modes.

The ActionBar has also been extended when in overlay mode so
that it will adjust the system window insets to also account
for its space, and allow it to be hidden using the new
SYSTEM_UI_FLAG_FULLSCREEN.

Change-Id: Ic8db1adec49a0f420bfe40c1d92eb21307856d0b