systemtray: Check if a service is already added before processing QDBusReply

Due to async nature of QDBusPendingReply, services could be already
registered by QDBusServiceWatcher when the pending reply takes a long
time to finish, so it's possible that QDBusServiceWatcher::serviceRegistered
signal is emitted before the pending reply emits QDBusPendingCallWatcher::finished,
which will make the same service added twice and crash plasmashell.

We need to check if a service is already added in m_sniServices before
processing registered items in QDBusReply.

BUG: 443961


(cherry picked from commit c0b8f687)