ApplicationPlayer: Sanitize ClosePlayer() and SetPlaySpeed()

After merging https://github.com/xbmc/xbmc/pull/2896, there was a 4/5 chance for a deadlock in CApplicationPlayer::GetInternal() when stopping video playback if a Python addon constantly polled the HasVideo bools.

When sanitizing SetPlaySpeed() and most importantly ClosePlayer() to also use the shared lock, the problem is gone.

Thanks to @t-nelson for the right hints and pointers to get this fixed!