Recorder: Add notification channels

Apps targeting API 26 (Android O), must send notifications through
"channels".

Create a channel in both sound and screen recording services, each one
aggregating the following notifications:
 - persistent notification(s) - eg. "xxx is being recorded"
 - recording completed notifications

Also create a channel for screen recording overlay, when it's ready
to start.

This way, a user can turn off particular channels if they don't want
certain types of notifications from Recorder.

Change-Id: I35301f8c300d05f6ba15ab769878a2ac4d4924e7