Advanced Audio Coding (AAC) is a lossy audio codec, which implements
a number of audio encoding tools and provides good audio quality
at relatively low bitrate. It is standardized as ISO/IEC 13818-7 (a.k.a.
MPEG-2 Part 7) and ISO/IEC 14496-3 (a.k.a. MPEG-4 Part 3) specifications.
It has gained wide adoption, including in Bluetooth A2DP devices.

Bluetooth A2DP specification standardizes the following AAC profiles:

 - MPEG-2 AAC Low Complexity (AAC LC)
 - MPEG-4 AAC LC
 - MPEG-4 AAC Long Term Prediction (AAC LTP)
 - MPEG-4 AAC Scalable

The former 3 are supported by this new PulseAudio codec for both decoding
and encoding.

AAC decoder is implemented in ffmpeg/libavcodec as a built-in decoder.

AAC encoder is implemented in ffmpeg/libavcodec in two variants:

 - built-in encoder, which is available since ffmpeg 3.0 (we require
   ffmpeg 3.1 because it introduces the modern API for working with
   codecs)
 - optional encoder backed by libfdk-aac

The A2DP codec is...