Parse channel layout value from dec3 box. Pass it through audio stream
info. MPD builder forms audio channel configuration to signal ec-3
codec.
Specs: 1) ETSI TS 102 366 V1.3.1 Digital Audio Compression (AC-3,
Enhanced AC-3) Standard E.1.3.1.8.
2) DASH-IF Interoperability Points v3.0 9.2.1.2.
Issue #64
Change-Id: Ia2c22dd3d82e757ba5a88ba1de35c5d593f5005e