Fix ContentObserver unregister issue

[Cause of Defect]
In the ContentObserver callback procedure, the binder thread
has no sync mechanism with handler thread.
The ContentObserver#onChange method can get called after
ContentResolver#unregisterContentObserver called in the
very low probability.

Bug: https://issuetracker.google.com/issues/63154326
Test: manual - check https://github.com/nanjingdaqi/ContentObserverDemo
Change-Id: I0e3831eba7a9cc1a5bf0d30abe5dd0ec8259b0d4
Signed-off-by: daqi <daqi@xiaomi.com>

core/java/android/database/ContentObserver.java

@@ -193,6 +193,11 @@ public abstract class ContentObserver {
      */
     private void dispatchChange(boolean selfChange, Uri uri, int userId) {
         if (mHandler == null) {
+            synchronized (mLock) {
+                if (mTransport == null) {
+                    return;
+                }
+            }
             onChange(selfChange, uri, userId);
         } else {
             mHandler.post(new NotificationRunnable(selfChange, uri, userId));
@@ -213,6 +218,11 @@ public abstract class ContentObserver {
 
         @Override
         public void run() {
+            synchronized (mLock) {
+                if (mTransport == null) {
+                    return;
+                }
+            }
             ContentObserver.this.onChange(mSelfChange, mUri, mUserId);
         }
     }