DO NOT MERGE registerSoftApCallback Executor param

Updating WifiManager.registerSoftApCallback to now have a @NonNull
Executor as a parameter vs @Nullable.

Bug: 147671244
Bug: 145592261

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

Change-Id: I85e6194f067e451842c01733cca2ea12bd164036