Update FGS notification behavior

Allow FGS notifications to be individually dismissed bu users, but not
clearable by clear all. Apps still cannot dismiss their FGS
notifications.

Also cleaned up some code from a previous 'allow dismissal' project.

Bug: 201579707
Test: atest SystemUITest
Test: atest NotificationManagerServiceTest
Test: flip flag, clear all does not dismiss a FGS but individual
swipe does
Test: without flag FGSes are not dismissible
Test: verify swipe/dismiss behavior for notif with FLAG_NO_CLEAR
Test: verify swipe/dismiss behavior for notif with FLAG_ONGOING_EVENT

Change-Id: Ibf9f55df400b7e9984d3fda5e6959878474f9886