audio: Disable sonification on digital USB outputs

 * We don't want to hear touch sounds on our badass USB DACs. Just
   play the damn music, please :)

Change-Id: I08b1c5d672d853fc7116c03cd77b8828881d3a5b
audio: fix offload enabling test parameter

passing NULL as the default to property_get doesn't return false,
it returns random memory junk that evaluates to true... Explicitly
default the state of this property to "false" as a default

Change-Id: Ib2808bf2540b2b4fdf754d489fb44aa5d9e061a8