SoftApCallback using executor vs handler

Updating classes to support making SoftApCallback methods available to @SystemAPI including
the API for registration of a SoftApCallback which includes a change to
now use executor vs handler.

Bug: 143564153
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherPreferenceControllerTest
Also tested manually on a Hawk.

Change-Id: I952ad10dd88b7d97b3cd86b269b8f67b00b1fa15