[power] make actual power off disabled unless overridden

<poweroffhdmi>true</poweroffhdmi> in advancedsettings.xml will enable
[omxplayer] Skip picture attachments in music files they cause audio to hang
[omxplayer] Use media for determing audio delay and cache time

I've also added caching to the call to OMXMediaTime as the GPU round trip is expensive when called too frequently
- before doing the real airplay service zeroconf announcement announce the same thing with different friendlyname - odd try to fix ios7 clients