Recorder: don't allow multiple instances of the app

Notifications could launch several instances of the app when clicked.
While this could have been fixed in other ways, we want to be 100%
sure the app doesn't have 2 open instances to prevent race conditions
when starting / pausing the record

Change-Id: I712b0ae711b11ee55c980d60cb730b2ace56fade
Signed-off-by: Joey Rizzoli <joey@lineageos.org>