screenrecord: Add audio support

Adds audio recording to the screenrecord system. Note that
while recording, the audio is muted from the device output,
and that the audio volume is controlled separately (so make
sure to turn up your volume if you hear nothing).
Audio from the microphone can also be recorded by adding
--microphone to the command line.

Change-Id: I97dfcaf3d93b786c5466e1f4215549e3d015bcf0
Merge "screenrecord: Add audio support" into android-4.4