Use HashSet to hold keys in Keyboard

This partially reverts Ie0554362.

Change-Id: I0de971c5efb4e8eb093be2a0793a08bd79baee22

java/src/com/android/inputmethod/keyboard/Keyboard.java

@@ -41,6 +41,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 
@@ -225,7 +226,7 @@ public class Keyboard {
         public int GRID_WIDTH;
         public int GRID_HEIGHT;
 
-        public final ArrayList<Key> mKeys = new ArrayList<Key>();
+        public final HashSet<Key> mKeys = new HashSet<Key>();
         public final ArrayList<Key> mShiftKeys = new ArrayList<Key>();
         public final ArrayList<Key> mAltCodeKeysWhileTyping = new ArrayList<Key>();
         public final KeyboardIconsSet mIconsSet = new KeyboardIconsSet();