AudioFormat: implement ENCODING_IEC61937

Allows applications to wrap compressed audio in a PCM
stream and pass through directly to HDMI output.

Bug: 24541671
Bug: 20891646
Bug: 26373761
Signed-off-by: Phil Burk <philburk@google.com>

Change-Id: If29e10b0665758330eca085bfeaac19fefc373b3