Allow IMEs to extend below nav bar, remove SystemUI veto.

Layout IMEs below the nav bar, offset by bottom padding and
associated guard rectangle with a black background to ensure
they do not appear as islands during transitions.

This makes it safe to remove the SystemUI forced opaque transition
when showing an IME, making the overall transition less expensive,
quicker and smoother overall.

Bug:11058746
Change-Id: I460912ee7c117480c57b947ed31eca330819f32c