Use TetherEnabler in AllInOneTetherSettings to manage master switch

In AllInOneTetherSettings, we use TetherEnabler to hanle all behavior
related to tethering switch on/off.
In TetherEnbler, add WifiManager.WIFI_AP_STATE_CHANGED_ACTION to cover
all possible tethering state change.
TetherEnablerTest is modified accordingly.

Bug: 147322704
Test: make RunSettingsRoboTests ROBOTEST_FILTER=CodeInspectionTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=TetherEnablerTest
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=AllInOneTetherSettingsTest

Change-Id: I505b3825f79260983fff9d3935ba834ad8f9f690