Refactor hotspot into a full page

- Deprecated WifiAPEnabler, and copied most of its logic into various
  controllers and WifiTetherSettings.
- Added tests

Fix: 37253404
Fix: 36181835

Test: make RunSettingsRoboTests
Change-Id: Iad994d61b694ad7f1113d045a3e7500eeaec178b