Updates to use flash notification settings keys in settings provider

Previously we hard-coded the settings key in FlashNotificationUtils. Change to use internal system settings key for retrieving flash notification settings from the database.

Bug: 266775683
Test: make RunSettingsRoboTests ROBOTEST_FILTER=CameraFlashNotificationPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=ScreenFlashNotificationPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FlashNotificationsPreviewPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FlashNotificationsUtilTest
Change-Id: I1e7a0c4ed58505cfb3c06e43af866744fa1b2d41