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
diff --git a/include/input/KeyCharacterMap.h b/include/input/KeyCharacterMap.h
index 867a089..b5e6f65 100644
--- a/include/input/KeyCharacterMap.h
+++ b/include/input/KeyCharacterMap.h
@@ -87,6 +87,9 @@
     /* Combines this key character map with the provided overlay. */
     void combine(const KeyCharacterMap& overlay);
 
+    /* Clears already applied layout overlay */
+    void clearLayoutOverlay();
+
     /* Gets the keyboard type. */
     KeyboardType getKeyboardType() const;