Made AccessPoint's scan result list thread safe

If wifi verbose logging is enabled, the main thread may iterate through
the scan result list while the worker thread updates it, resulting in
an ArrayIndexOutOfBounds exception. This change adds a lock to every
iteration through mScanResults and mExtraScanResults.

Bug: 134098126
Test: atest WifiTrackerTest && atest AccessPointTest
Change-Id: If42a9ad8031ff66b1732a79772ba441185f7a18f