Do not assume lipstick D-Bus service can handle compositor interface

Starting from lipstick-qt5 0.24.7, the compositor interface is
served from org.nemomobile.compositor D-Bus name and mce no longer
needs to support legacy behavior where it could be provided via
org.nemomobile.lipstick D-Bus name too.

[mce] Do not assume lipstick D-Bus service can handle compositor interface. Fixes JB#19572