Add system exemption for dismissible notifications

* Starting from Android U, notifications with the ongoing parameter
  can be dismissed by a user on an unlocked device unless the
  appl that created the notification is exempt.
* Introduce the appop
  OP_SYSTEM_EXEMPT_FROM_DISMISSIBLE_NOTIFICATIONS. An app with this
  appop will be made exempt.

* This new appop is replacing an appop in Android U that was unused.

Bug: 246330879
Bug: 266237746
Test: atest AppOpsTests
Change-Id: Idb7997ae21b812b537400d699405ad148e99c6b3