update to buildToolsVersion 21.0.2
Fix 2 bugs arising from granting AlarmManager a time window.

AlarmReceiver schedules the next repeating alarm in [-window .. 0] of the
next repeating alarm time. This lets AlarmManager save battery power but
has subtle implications.

1. The notification's alarm count display was 1 low when Notifier runs a
little early to construct the notification.

2. The notification and its alarm sound can get restarted mid-stream when
rescheduling happens early enough to happen again before the actual alarm
time.