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>