Marking SoftApCallback methods as SystemAPI

Making SoftApCallback methods available to @SystemAPI including
the API for registration of a SoftApCallback. This includes updating
registration to use an executor now vs a handler.

Bug: 143564153
Test: atest FrameworksWifiApiTests:android.net.wifi.WifiManagerTest
Also tested manually on a Pixel 3.

Change-Id: I22b5029a8dbd7a50ad6faf0bb3b15269af839956
Merged-In: I22b5029a8dbd7a50ad6faf0bb3b15269af839956