[non-BE][split-non-BE] Resolve "Match loudness for stereo and mono in ParamUpmix mode"

• Related issues: #787 (closed)
• Requested reviewers: schug

Reason why this change is needed

Match loudness of Mono/Stereo rendering in ParamUpmix with other modes

Description of the change

Apply gain to upmix channels.

Affected operating points

7.1.4 160kbps when decoding to Mono or Stereo, channels other than first 4 will be louder.

Bitstream should be unchanged

Produced decoded output is non-BE but is fully backwards compatible for decoding.