Refactor the mechanism for delivering periodic callbacks

Bug: 25983208
Bug: 26381960

Periodic timed callbacks are used to update portions of the UI that
may change at time boundaries, such as date displays when midnight
passes. The old mechanism required the API user to define their own
Handler and also compute and schedule each callback. Also, no
corrective rescheduling occurred if the device time changed.

The new design introduces PeriodicCallbackModel as a delegate to
UiDataModel. The delegate removes all of the burdens associated
with scheduling and canceling period callbacks. The caller need only
supply the Runnable that contains the logic to execute and an offset
in millis if they want to be called before or after the period ends.

Change-Id: Ibb51b22af081fc4df60ee1c561f2c9aaf1ddef96