Fix unexpected crashed in IncomingCallNotifier

mCalls in IncomingCallNotifier is not synchronized but 2 threads acces it at the same time.
Test: Manually test
Bug: 150799582

Change-Id: I68ffe7069d6c23f4799d41f2f9457b4d87f878b6
Signed-off-by: Taeun Kim <taeun99.kim@samsung.com>