[AE/PulseAudio] fix for volume, device selection

first commit is to fix pulsaudio volume step under xbmc. This is caused by volume range difference between AE and PulseAudio and as a result the minimum volume (apart from silent) you will get with XBMC is 10% or -59.33 dB.

second commit is to fix sample file playback constant 100% volume, aka skin/menu sound. The fix brings menu sound volume to the same level as everything else.  

third commit is to fix xbmc gui audio devices (including passthrough) are being ignored. If user set it then system should use it otherwise what the point having it in the first place?