Refactor Timers to use new unified DataModel

Bug: 24816218
Bug: 25494722

This refactoring introduces a Timer domain object. Instances of these
domain objects are fetched via a DataModel class that is the single point
of reference for all timer data. This allows the data to be shared across
the entire application. In particular, the timer notifications, TimerFragment
and ExpiredTimersActivity all draw upon the same model to determine
what the state of the timers are.

IMPORTANT: The public intent com.android.deskclock.action.STOP_TIMER
has been renamed to com.android.deskclock.action.PAUSE_TIMER to be
more accurate.

Change-Id: Ib904e80a25c6de1861de2b395bec27db16ff0e66