Optimize KeybaordView.invalidateAllKeys a bit

Change-Id: I118c792219f649b7297477c78fa725c678650159
diff --git a/java/src/com/android/inputmethod/keyboard/KeyboardView.java b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
index a174fd9..abc220e 100644
--- a/java/src/com/android/inputmethod/keyboard/KeyboardView.java
+++ b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
@@ -944,6 +944,7 @@
     public void invalidateAllKeys() {
         mDirtyRect.union(0, 0, getWidth(), getHeight());
         mBufferNeedsUpdate = true;
+        mInvalidatedKey = null;
         invalidate();
     }