Check group channels for FGSes

Before allowing the group to be deleted, by updating
the current check to the method that populates the channel
list

Test: NotificationManagerServiceTest
Bug: 209965481
Change-Id: I9db781c300e96e9c80bd5d21585b8be9b4db08c8
Merged-In: I9db781c300e96e9c80bd5d21585b8be9b4db08c8