Fix TimeTicker not registered when unregistering

Unregistering the `TimeTicker` could happen multiple times, and it's
happening during onDestroy. It might potentially assign ticker
without unregistering, or unregister the same ticker multiple times as well.

To fix these:
1. The unregister is happened when `onDestroy`, changed to `onPause`.
2. Prevent multiple calls to register/unregister by explicit
   registering/unregistering.

Fixes: 269196790
Test: tapping the settings entry fast
Change-Id: Ie162a095ba81ba02d24ee7225576bffc554bf27c