media-keys: Fix mmkeys D-Bus API to match API docs

Bizarrely, since 2011, gnome-settings-daemon was documented as using
org.gnome.SettingsDaemon.MediaKeys D-Bus name, but everybody ended up
using the org.gnome.SettingsDaemon owned by the daemon instead, and
never reported the discrepancy.

This fixes the code to match the 6-year old API as documented by owning
the org.gnome.SettingsDaemon.MediaKeys as well as the
org.gnome.SettingsDaemon D-Bus name to give 3rd-party users time to
adapt their code.

The portion of this patch adding the org.gnome.SettingsDaemon.MediaKeys
name owning will need to be backported as far as reasonably possible by
distributions, and all users of the API changed before GNOME 3.26. This
would obviously have been easier if the problem was reported when
detected, committer of this fix included.

https://bugzilla.gnome.org/show_bug.cgi?id=781326