Fix WifiTracker keeps sacnning even if onStop() is called.

In critical case, WifiTracker.pauseScanning() and updateWifiState() are called
in same time but in different threads, causes racing condition problem.
mScanner.resume() will be called after mScanner.pause() but before  mScanner
is set as null.
Adding synchronized to prevent it.

Bug: 123404912
Test: atest WifiTrackerTest
Change-Id: I3a24fb0830cd1d0c93eca03786c10a1d168de00e