Avoid refreshing world clocks when none are selected

Bug: 26063240

DigitalAppWidgetProvider happily schedules callbacks with AlarmManager
that occur every quarter hour. Those callbacks refresh the week day
associated with every selected world clock every 15 minutes (see Nepal's
timezone for why it is 15 minutes).

This CL avoids scheduling a quarter hour callback in the common case: the
user has not selected any world clocks for display.

Change-Id: I7e4234d75e9301d000b5aac52c88c022061c348a