WifiScanner: retrieve single scan results

Add a new CMD_GET_SINGLE_SCAN_RESULTS message type and
getSingleScanResults method that allow WifiService to get
scan results directly from WifiScanner.

BUG: 31444878
Test: manually tested on angler
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh on angler
Test: ran GTS CtsNetTestCases on angler
Change-Id: I705233d7dc4ae7e8480c53ff0f9e1b6c8875fecb