Adjust divider when IME height is changed.

Previously, divider was adjusted for IME when IME window visibility is
changed. However, IME size can be changed without changing IME window
visibility. For example, when hardware keyboard key is pressed to show
emoji layout.
This CL checks if IME height has been changed and adjust divider position
properly.

Bug: 28389933
Change-Id: I9a561bd4796a0513c8b5c493884946883f160907