Enterprise exemption for dismissible notificatons

Starting from Android U, notifications with the ongoing parameter
can be dismissed by a user on an unlocked device. An exempted app
can have non-dismissible notifications.

The following exemptions will be added:
* Admin apps
* Apps that are granted the SYSTEM_EXEMPT_FROM_DISMISSIBLE_NOTIFICATIONS

Allow apps with the MANAGE_DEVICE_POLICY_APP_EXEMPTIONS permission
can grant the SYSTEM_EXEMPT_FROM_DISMISSIBLE_NOTIFICATIONS exemption
to other apps.

Bug: 246330879
Bug: 266237746
Test: atest NotificationManagerServiceTest
Change-Id: Iff0b785e096c22dd466401c7af14964e04cc73da