Some channel model updates

- Apps cannot update their channel settings after creation.
- Importance is required when creating a channel.
- Some method name changes.
- Ranker can't modify fields a user has changed.
- High and Max importance mean the same thing.
- The default channel adopts app wide settings on creation.
- The default channel is limited to importance low once target api is post n mr1
  unless the user changed it.

Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/notification

Change-Id: I73c449a6abe6d709046de79c5c54339cb2edf0b8