[pvr] fix built-in PlayMedia with PVR channels and recordings (Trac #15164)

This should fixes the playback of PVR channels and recordings with the built-in PlayMedia as reported at http://trac.kodi.tv/ticket/15164
@opdenkamp the problem right now is that if we return false, `CApplication` will never get to the last line. That needs to be changed though.

Regarding your other comment, is it actually necessary to avoid the application messenger if you're already on the main thread?