AS-11 X9: Correct AAC audio issues

[Matthew-Marks]

1. The Audio Layout Mode Block states that channel assignment is signalled by the "ChannelAssignment" property of the {WAVEPCMDescriptor}. This is not the case for AAC audio, where it is the "MPEGAudioChannelAssignment" property of the {MPEGAudioDescriptor}.
2. The Unconstrained ST 381-4 AAC mapping Block does not mandate a channel assignment property (compare with Constrained ST 382 Wave PCM mapping Block)
3. The "ElectrospatialFormulation" property of the Sound Essence Descriptor should be forbidden for AAC as well as PCM audio, as both audio descriptors support this property (see Constrained ST 382 Wave PCM mapping Block).

Remedies:

1. Update Audio Layout Mode to give a format-agnostic description of the requirement, and specify what needs to be done for PCM and for AAC in a Note. (This Block is used elsewhere in the X family, so it is better to confine references to AAC to a Note as it only used in X9 at present.)
2. Rename Unconstrained ST 381-4 AAC mapping to Constrained ST 381-4 AAC mapping and add the two Artifacts referring to channel assignment from Constrained ST 382 Wave PCM mapping.
3. Add the Artifact referring to "ElectrospacialFormulation" from Constrained ST 382 Wave PCM mapping to Constrained ST 381-4 AAC mapping.

Consequences:

• No changes to compliant files with PCM audio; files with AAC audio will require their MPEGAudioDescriptors to have an MPEGAudioChannelAssignment property present and as per the Specification, and the ElectrospacialFormulation property must be absent
• The wording of other Specifications will change, but there will be no material differences.

[Matthew-Marks]

Specifications updated.