Prevent kdeconnectd delays from affecting plasmoid startup

Adding protection around trying to start the service if it isn't registered. Without this, the synchronous call to start() can cause severe delays when plasma loads if kdeconnectd is very slow to startup.

Other half of !643 (merged)

Tested by not applying 643 and having no bluetooth, this resulted in no 25 second pause on startup; then also tested with working paired configuration and does not appear to have any negative effects in my testing.