Move expensive TextClock operations to onAttach

Move expensive TextClock operations to onAttach

Registering and unregistering are expensive operations
that should not be done when the visibility of the
TextClock changes. However, we want to ensure that a
non-visible TextClock does not consume extra resources
or cause layout/redraw passes. By keeping the TextClock
registered but avoiding the side effects of onTimeChanged,
we can get the best of both worlds.

Test: manual testing

BUG: 33960344
Change-Id: I3e76246d820363766b78130231d665989fdb4265