[non-BE] fix for MSAN issue 778 for TNS in unified stereo

• Related issues: #778 (closed)
• Requested reviewers: @vaclav (possibly @vaillancour )

Reason why this change is needed

• MSAN error in unified stereo: for 24.4kbps switching between TD/DFT stereo may cause turning TNS off/on respectively. When TNS is turned on again after TD stereo TNS configuration is currently not being initialized.

Description of the change

• check whether there is a change of fIsTNSAllowed flag in current frame if st->element_mode==IVAS_CPE_DFT and if yes, set TNS configuration based on bitrate

Affected operating points

• non-BE for unified stereo 24.4kbps

Related to #778 (closed)