Check for SCHEDULE_EXACT_ALARM permission, somehow gracefully fall back

[UweTrottmann]

App is crashing trying to schedule notification alarms due to users turning off the Alarms & Reminders permission.

https://developer.android.com/training/scheduling/alarms#exact-permission-check

Potential solution:

• display warning in settings if exact alarm permission has been revoked.
• implement broadcast receiver to re-schedule alarms as indicated in above docs (though NotificationService runs frequently when using the app, so maybe not).