frameworks/base: Fix ContentService JNI leaks

- A list of ObserverCall(s) is created by ContentService as
  it notifies changes to the registered content observers.
  Clear observer after use.

- If same Observer and same URI are registered, the callback
  functions are called multiple times when data identified by
  a given content URI changes. It makes system slow and it
  cause ANR sometimes. And when run
  unregisterContentObserver(), just one deleted but others
  are remained in mObservers. So add prevent code to avoid
  to duplicated registration of Observers

CRs-Fixed: 868634

Change-Id: I5906f581f4beaed04af09e5b9e3999f67d94f1fc