IME always provides bottom insets, if any

This fixes an issue where IME wasn't visible in landscape with a
cutout, because IME window didn't cover full edge.

Also fix an issue where floating IME didn't work because alpha
was calculated the wrong way.

Test: InsetsSourceTest
Bug: 111084606
Change-Id: I29b7a45cd590a52ecb03f2e539edde57aef4dba3