getObjectTypeFromMp4aRFC6381CodecString

Returns the Mp4aObjectType of an RFC 6381 MP4 audio codec string.

Per https://mp4ra.org/#/object_types and https://tools.ietf.org/html/rfc6381#section-3.3, an MP4 codec string has the form:

~~~~~~~~~~~~~~ Object Type Indication (OTI) byte in hex
 mp4a.[a-zA-Z0-9]{2}(.[0-9]{1,2})?
 ~~~~~~~~~~ audio OTI, decimal. Only for certain OTI.
*

For example, mp4a.40.2 has an OTI of 0x40 and an audio OTI of 2.

@param codec An RFC 6381 MP4 audio codec string. @return The Mp4aObjectType, or null if the input was invalid.