mpris: correct mpris MediaPlayer2 interface names...

A-hem.

It's org.mpris.MediaPlayer2, not org.freedesktop.mpris.MediaPlayer2,
nothing sed -i -e 's/org\.freedesktop\.mpris\./org.mpris./g' *.c **/*c
cannot fix.

We also conveniently skip the opportunity to replace these open-coded
instances with a single constant.