Fix bug: "Hotspot & Tethering" preference doesn't get updated

Aissign the value of mPrefrence when displayPrefrence() being called.
This can fix the preference doesn't get updated bug. Added test to
ensure it.

Bug: 160169699
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=AllInOneTetherPreferenceController

Change-Id: If0e7ce421cc46cff2b92837d2d07ea0ff4dc5fd9