Fix the reference leak on RemoteCallbackList

In case that register is invoked consecutively with same callback,
reference leack could be happened since unlinkToDeath for all
callbacks is not being called.

Test: call register multiple times with same callback and unregister
callback same times. confirm the global reference table information.

Change-Id: I7dbf108ea87b3ee7ce1e1a0ff75e05e8c4478f67