Add placeholder onGrabbedStateChange() method.

Update LockScreen to implement the latest RotarySelector
OnDialTriggerListener interface.  In a followup CL we'll use this to
display onscreen hint text while dragging.

BUG=2158434
DRNO=timsullivan
TESTED=exercised the lock screen widget, confirmed there's no new behavior.

policy/com/android/internal/policy/impl/LockScreen.java

@@ -247,6 +247,11 @@ class LockScreen extends LinearLayout implements KeyguardScreen, KeyguardUpdateM
         }
     }
 
+    /** {@inheritDoc} */
+    public void onGrabbedStateChange(View v, int grabbedState) {
+        // TODO: Update onscreen hint text based on the new state.
+    }
+
     /**
      * Displays a message in a text view and then removes it.
      * @param textView The text view.
@@ -556,4 +561,3 @@ class LockScreen extends LinearLayout implements KeyguardScreen, KeyguardUpdateM
         mUpdateMonitor.removeCallback(this);
     }
 }
-