Prevent adding duplicate callbacks to the state controller

- KeyguardBottomAreaView can add itself multiple times as a callback,
  which can leak a bitmap via that view if the state controller keeps
  an extra ref to it.

Bug: 141555865
Bug: 140629244
Test: Change overlays multiple times, take heap dump and ensure no
      leaked bitmaps

Change-Id: I8169eac24d8801ae71e4bdc60a16c68eb3896cc4

packages/SystemUI/src/com/android/systemui/statusbar/policy/KeyguardStateControllerImpl.java

@@ -101,7 +101,9 @@ public class KeyguardStateControllerImpl extends KeyguardUpdateMonitorCallback
     @Override
     public void addCallback(@NonNull Callback callback) {
         Preconditions.checkNotNull(callback, "Callback must not be null. b/128895449");
-        mCallbacks.add(callback);
+        if (!mCallbacks.contains(callback)) {
+            mCallbacks.add(callback);
+        }
         if (mCallbacks.size() != 0 && !mListening) {
             mListening = true;
             mKeyguardUpdateMonitor.registerCallback(this);