[non-BE] Resolve "OSBA rendering to Mono and Stereo for <96kbps is significantly quieter"

• Related issues: #1518 (closed)
• Requested reviewers:

Reason why this change is needed

• Loudness is inconsistent between bitrates with stereo output from OSBA input

Description of the change

• Make the sacling in the decoder conditional on the ISM mode

Affected operating points

• Output is non BE in low-BR OSBA-to-stereo decoding

Related to #1518 (closed)