avahi: Style and code cleanups in zeroconf module.
avahi: Style and code cleanups in zeroconf module.

- Prepare for handling missing dbus directly in module.
- Constants should always be all caps.
- Extracted helpers from class to convey intent via their naming.
- Moved imports out of class, imports should always be on the top.
- Made sure calling publish mutiple times does not re-convert text.
- Made sure calling unpublish without publish does not break.

bacontext/mopidy,bacontext/mopidy,priestd09/mopidy,dbrgn/mopidy,adamcik/mopidy,bencevans/mopidy,jmarsik/mopidy,jcass77/mopidy,swak/mopidy,abarisain/mopidy,mopidy/mopidy,tkem/mopidy,glogiotatidis/mopidy,liamw9534/mopidy,ZenithDK/mopidy,jodal/mopidy,quartz55/mopidy,jmarsik/mopidy,bencevans/mopidy,jmarsik/mopidy,glogiotatidis/mopidy,mokieyue/mopidy,rawdlite/mopidy,SuperStarPL/mopidy,ali/mopidy,vrs01/mopidy,priestd09/mopidy,hkariti/mopidy,dbrgn/mopidy,jodal/mopidy,jmarsik/mopidy,tkem/mopidy,bacontext/mopidy,mokieyue/mopidy,quartz55/mopidy,bencevans/mopidy,ZenithDK/mopidy,bacontext/mopidy,swak/mopidy,tkem/mopidy,quartz55/mopidy,abarisain/mopidy,glogiotatidis/mopidy,bencevans/mopidy,mopidy/mopidy,quartz55/mopidy,woutervanwijk/mopidy,jcass77/mopidy,kingosticks/mopidy,dbrgn/mopidy,tkem/mopidy,pacificIT/mopidy,adamcik/mopidy,dbrgn/mopidy,kingosticks/mopidy,adamcik/mopidy,jodal/mopidy,hkariti/mopidy,glogiotatidis/mopidy,priestd09/mopidy,swak/mopidy,swak/mopidy,kingosticks/mopidy,SuperStarPL/mopidy,liamw9534/mopidy,rawdlite/mopidy,rawdlite/mopidy,ZenithDK/mopidy,vrs01/mopidy,pacificIT/mopidy,ali/mopidy,ali/mopidy,diandiankan/mopidy,SuperStarPL/mopidy,ZenithDK/mopidy,jcass77/mopidy,pacificIT/mopidy,vrs01/mopidy,mopidy/mopidy,hkariti/mopidy,woutervanwijk/mopidy,mokieyue/mopidy,mokieyue/mopidy,ali/mopidy,diandiankan/mopidy,rawdlite/mopidy,diandiankan/mopidy,hkariti/mopidy,pacificIT/mopidy,SuperStarPL/mopidy,diandiankan/mopidy,vrs01/mopidy