Multi cutout: Fix IME navigation guard

Ensures that the IME window always extends all the way to the bottom.
This is fine because in its decor view the navigation bar inset is
consumed by the navigation guard.

Bug: 74195186
Test: Enable multi-cutout; open a text field over non-black navigation bar. Verify entire bar turns black.
Change-Id: Iff142ccd2034684c37e140e4ccd1839691dbf443