[Settings] Enabling first stage of threading

Settings app would move PreferenceController to run
in background thread in the near future.

To support this, change some of the design within these
essential components to allow:
1. Handler to run on non-main thread
2. Minimum multi-thread reentrance support

Bug: 146045802
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=GlobalSettingsChangeListenerTest
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=ActiveSubsciptionsListenerTest
Change-Id: Ica5c942b919007d7d260feee632752d3353a6f18