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: I80771524a09ca57488834bafd97b570f2a8235db