Clear applied layout overlay if new layout overlay is null.

Need to allow resetting of key character map by setting overlay
map to null. This is required when moving from a IME setting
that has a KCM to another IME setting that has none.

Test: atest KeyboardLayoutChangeTest
Bug: 20805588
Change-Id: I62dc65970b61628486aff698a8ab8ccb31c109d6