Allow users to hide notifications from foreground services.

Since Android 4.3 foreground services get a generic notification from the
system if they provide an invalid one. This is done to prevent services from
running in foreground without the user knowing. However, if the user is aware
of the service and doesn't want to see the notification anyway (like it used
to be) he should have the possibility to hide it.

There is already the possibility to deactivate notifications from certain
apps. Coincidentally this already allows to clear notifications from
foreground services. However this didn't prevent new notifications from
appearing. This should be possible with this change.

Change-Id: I6bbcbc8452385abbd09105c73a7c8aca4360c39c