Allow the user to block notifications for foreground services.

Stepping carefully around the fix in change Idc788527, we
perform security (and AppOps, and therefore user block)
checks against the uid passed to enqueueNotificationInternal
rather than the Binder caller.

Bug: 9502541
Change-Id: I91745db92823381df75721c1b8f11013a7fa4ecc