Only make one request for list of MPRIS players during plugin creation

Summary:
MPrisPlugin previously made two calls to requestPlayerList() in its
onCreate() method. This change deletes the first of those, so that
only one network packet is sent and received.

Test Plan:
Pair a new device, confirm that only one request for media players is
received by remote client.

Reviewers: KDE Connect, mtijink

Reviewed By: KDE Connect, mtijink

Differential Revision: https://phabricator.kde.org/D11648