Group screen record "error starting" notifs and "error saving" notifs.

This ensures that the notification title will always say "Screen
Recorder", and never "System UI".

Fixes: 356586297
Flag: EXEMPT bugfix

Test: Start and finish multiple successful recordings, and also start
and finish multiple failed recordings (e.g. choose to record a single
app, but close that app during the countdown and never reopen it) ->
Verify all "recording saved" notifs are grouped together, and all "error
saving" notifs are in a separate group. Verify the active "recording"
notif is always on its own.
Test: Have a failed recording, then switch users -> verify notif doesn't
appear
Test: atest RecordingServiceTest

Change-Id: I18922eb9f12d8ed92592c777fe8102325a312a37