[Screen Record] Assign fixed IDs to screen record notification groups.

Previously, the screen recording group notifications were using the ID
of one of the *individual* notifications. This meant that if you swiped
the group away, it actually dismissed just an individual notification,
*not* the whole group, because the group ID was equal to that individual
notification.

This CL instead uses fixed IDs for the group notifications so that
dismissing the group does indeed dismiss the *whole* group, not just a
single notification within the group.

Fixes: 368837832
Flag: EXEMPT BUGFIX

Tests to verify b/368837832 is fixed:
Test: Start and finish multiple successful recordings, then swipe away
the whole "Recording saved" group -> verify whole group is dismissed
and individual notifications aren't re-shown
Test: 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), then swipe away the whole "Recording failed" group -> verify whole
group is dismissed and individual notifications aren't re-shown

Tests to make sure Change-Id I18922eb9f12d8ed92592c777fe8102325a312a37
didn't regress:
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: I32450067933eb1250f8d149a6445a3d0d8257cea