Bug fix of NPE in isTimeoutScanClient()

- Uses a synchronized method to check isTimeout

Tag: #stability
Bug: 243321425
Bug: 243611867
Bug: 244124045
Test: atest BluetoothInstrumentationTests:ScanManagerTest
Ignore-AOSP-First: T-QPR1 change

Change-Id: Ic8e60c1963add34afe79d13de65f6b72cfeef1be
Merged-In: Ic8e60c1963add34afe79d13de65f6b72cfeef1be