Initialize ContentObserver in startObserving().

This removes a race condition where ContentObserver is initialized
while mHandler is null which causes the ContentObserver#onChange() callback to run on a different thread and can cause an NPE.

Bug: 246843362
Test: manual
Test: atest BrightnessSynchronizerTest
Change-Id: Ib8e192c3961390ece721fa473d5c09323c30bafe

core/java/com/android/internal/display/BrightnessSynchronizer.java

@@ -115,7 +115,7 @@ public class BrightnessSynchronizer {
             Slog.i(TAG, "Setting initial brightness to default value of: " + defaultBrightness);
         }
 
-        mBrightnessSyncObserver.startObserving();
+        mBrightnessSyncObserver.startObserving(mHandler);
         mHandler.sendEmptyMessageAtTime(MSG_RUN_UPDATE, mClock.uptimeMillis());
     }
 
@@ -482,7 +482,8 @@ public class BrightnessSynchronizer {
             }
         };
 
-        private final ContentObserver mContentObserver = new ContentObserver(mHandler) {
+        private ContentObserver createBrightnessContentObserver(Handler handler) {
+            return new ContentObserver(handler) {
                 @Override
                 public void onChange(boolean selfChange, Uri uri) {
                     if (selfChange) {
@@ -493,19 +494,19 @@ public class BrightnessSynchronizer {
                     }
                 }
             };
+        }
 
         boolean isObserving() {
             return mIsObserving;
         }
 
-        void startObserving() {
+        void startObserving(Handler handler) {
             final ContentResolver cr = mContext.getContentResolver();
-            cr.registerContentObserver(BRIGHTNESS_URI, false, mContentObserver,
+            cr.registerContentObserver(BRIGHTNESS_URI, false,
-                    UserHandle.USER_ALL);
+                    createBrightnessContentObserver(handler), UserHandle.USER_ALL);
-            mDisplayManager.registerDisplayListener(mListener, mHandler,
+            mDisplayManager.registerDisplayListener(mListener, handler,
                     DisplayManager.EVENT_FLAG_DISPLAY_BRIGHTNESS);
             mIsObserving = true;
         }
-
     }
 }