Flash notification-related APIs should be restricted to system level access only

Flag: EXEMPT bugfix
Bug: 312197121
Test: tested with the attached APK in buganizer and confirmed that flash notifications are not displayed.
Test: atest AccessibilityManagerTest#testStartAndStopFlashNotificationSequence
Change-Id: I5d593a1bc5eaeb29023e09cc785633eb394b7034