Added volume support on android

Volume is only being used on IOS, but ignored on android.

Also, the parameter lists for LowLatencyAudio is inconsistent between android and iOS for the preloadAudio function. On Android the third parameter is voices but on iOS the third parameter is volume.

This pull request addresses the above and sets the signature for the preloadAudio function to:
  function(id, assetPath, volume, voices, success, fail)
for both android and iOS.

BTW, many thanks for your excellent work on both this and the AdMob plugin for cordova. Really appreciated.