Avoid ANR of TetherPreferenceController

Off load the following work from main thread,
- Calculate title
- Calculate summery

This also helps improve the latency.

Also migrate to registerTetheringEventCallback() since
TetheringManager.ACTION_TETHER_STATE_CHANGED is deprecated.

Fix: 311848767
Test: manual - on Network & internet page and turn on / off tethering
Test: unit tests
Change-Id: I6ee182b41ef51f691ea31938142be1a41faf5573