Do let IME fit invisible insets

Otherwise, the fullscreen IME won't cover the entire display while
there is a hidden system bar.

Fix: 168657591
Test: 1. Steps in the bug.
      2. Play WindowInsetsTests with combinations of system bar
         visibilities, navigation mode, screen orientation, and display
         cutout.
Change-Id: I6e7d665c55839dfbb14c8d2e5365537416f5f6c6