Disable digital noise gate (fixes CYAN-4506).

This fixes CYAN-4505 (Sound cuts out at low volume) [1].
One might think that the digital noise gate (DNG) is better left enabled to
reduce the static noise but in my tests, only the "payload" audio was cut off,
while the static continued. For me it seems that this feature of the sound chip
was not intended for use in a smartphone.

[1]: https://jira.cyanogenmod.org/browse/CYAN-4506.

Change-Id: I4e41c594695f85574ed35e2aef59d6b768403606
Merge "Disable digital noise gate (fixes CYAN-4506)." into cm-11.0