Add toString() method to Keyboard

Change-Id: I74c8f006323ed78ec67414fd17849ab40966968e

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

@@ -219,6 +219,11 @@ public class Keyboard {
         return code >= CODE_SPACE;
     }
 
+    @Override
+    public String toString() {
+        return mId.toString();
+    }
+
     public static class Params {
         public KeyboardId mId;
         public int mThemeId;