Reset Divider IME state when leaving split-screen

This is basically a broad-stroke reset after leaving
split-screen to clean-up ime state inconsistencies. Generally
this won't really do anything, but it should hopefully
prevent bad state from persisting when it does happen.

Bug: 161155771
Test: Use split-screen stuff. It should continue working.
Change-Id: If225dee39b8eca160a32a0953d23ebe86cceff9e