Allow package verifier to hide specific fgs notifications.

The package verifier performs some system critical work that the user
doesn't need to be explicitly aware of. This adds a mechanism for the
verifier to indicate which foreground service notifications should be
hidden, if possible.

Bug: 164440539
Test: use test app to confirm requested fgs notifications are hidden
Change-Id: Ib3eb0b71cc676c145557ade9def98a363e5abebb