Prevent a duplicated registration of Observers

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 registraion of Observers

Change-Id: I5c7845deb682db289ff0d53150ef012174859767
Signed-off-by: muzbit.kim <muzbit.kim@lge.com>