Remove hidden onAvailable callback override

ConnectivityManager will be a part of incoming connectivity
mainline. Settings will no longer to access the hidden usage.
The usage in WifiStatusTracker should only need to do in the
onCapabilitiesChanged but not to refer to a hidden callback.
Replace the usage to remove the hidden dependency.

Bug: 172183305
Test: Verify the behavior works as expected
Change-Id: I1bdc1c88869ebf815117ceccd2b008681e8290c5