Implement client-side unregister_notification command

Clients can currently register for notifications from bt-daemon when
a particular key is changed. This commit adds support for opting out
of those notifications, via the unregister_notification command.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
daemon: Support unregister_notification

We now support unregistering for previously registered notifications.
If the client requesting removal of notification is the last one to do
so, we also remove the mapping from our known notifications to ensure
we don't worry about it on the next send out of notifications.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
test/check_daemon: Add checks for unregister_notification

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
test: Add valid pointers to smack labels

Since we don't check the labels because only groups are being used for
these tests, the data doesn't need to be initialized but we still need
valid pointers to avoid a possible assertion failure.

Signed-off-by: William Douglas <william.douglas@intel.com>