Allow alarms to be modified by 3rd parties.

- Export the ClockProvider.
- Move the ClockContract to the CM Platform SDK for external reference.
- Add three new permissions:
  - "cyanogenmod.alarmclock.permission.WRITE_ALARMS" - This
    system|signature only permission gives full write access to the
    ClockProvider.
  - "cyanogenmod.alarmclock.permission.READ_ALARMS" - Read only access to
    the ClockProvider.
  - "cyanogenmod.alarmclock.permission.MANAGE_ALARMS" - Access to perform
    a subset of all alarm changes such as turning on or off alarms.
- Protect the provider with the above permissions for read/write.
- Move the logic for updating an Alarm object directly into the Alarm
  class.
- Add a new IntentService to allow turning on and off alarms when
  started by an application with the MANAGE_ALARMS permission.

Change-Id: I399ce9f1073c6fdcb1711a11095f7c3a4bce2c6a