kio-mtp: register demon on D-Bus by service name org.kde.kmtpd5

Currently nothing tries to address the service via the well-known name,
but in case someone starts to do so, fixing X-KDE-DBus-ServiceName in the
JSON metadata to match the name used in the D-Bus service file
should ensure things work as expected.
Using org.kde.kmtpd5 as well-known service name matches the pattern
seen elsewhere with KDE services.