WifiConfig: Hide RecentFailures and expose int failure code

RecentFailures is just a wrapper around an int,
no need to expose it. Instead, just expose a
getter for the int itself, and the constants
for the possible values of the int.

Bug: 146046526
Test: make RunSettingsLibRoboTests -j40
Test: atest FrameworksWifiApiTests
Change-Id: I817ec208c5ac6658c9d6cf443837ff7ea82679e0