Use string notifications_disabled instead of off

notifications_disabled was changed to off in
Change: I5f84d09f223efd478461ded93aeac82bf7b128d8

And reverted from off to notifications_disabled in
Change: Iaad301c5513478fb95e40987ea3ccb4f923d71fa

This usage is branched from AppNotificationPreferenceController.java,
and missed in revert.

Bug: 272603842
Test: Visual
Test: Unit test
Change-Id: I5b8466134c1342e10de7a2ead8c52931e0c19377