DO NOT MERGE - Disallow deletion of channels with FGS notifications

Bug: 156090809
Test: atest CtsAppTestCases:NotificationManagerTest
Test: atest CtsAppTestCases:android.app.cts.ServiceTest
Change-Id: I1c2bb78d86f194585d273661cecf3419f51965df
Merged-In: I1c2bb78d86f194585d273661cecf3419f51965df
6 files changed