Introduce StopwatchListener to gracefully handle BOOT_COMPLETE

After a device reboot, there is a window of opportunity (about 15 seconds)
in which the user may open the Clock app and view the stopwatch. They
will see the stopwatch in a non-operational state. When BOOT_COMPLETE
is finally broadcast, the stopwatch is reset and the laps are cleared.

Prior to this change, the stale lap data would continue to be displayed
because no mechanism existed to notify StopwatchFragment of changes
to the stopwatch data by an external entity.

This change introduces StopwatchListener, which is notified of changes to
the stopwatch and its laps. The StopwatchFragment uses this listener to
react to stopwatch changes from external entities, such as the
BOOT_COMPLETE receiver.

Clearing laps is now a byproduct of resetting the stopwatch instead of an
independent update to the model.

Change-Id: Ie1aa3d3e5006d8a28947cb3cd6343175d1c3644c