[non-BE] Fix clicks in stereo switching

• Related issues: ivas-codec-pc/ivas-codec#1563 (closed)
• Requested reviewers: @vaillancour

Reason why this change is needed

• porting of ivas-codec-pc/ivas-codec!2577 (merged)
• Serious artifacts are observed in the bitrate switching which involves switching of different CPE modes.

Description of the change

• Set TD stereo secondary channel core to ACELP to avoid the core switching in this channel which causes serious artifacts.
• Rename stereo_switching_dec() to stereo_switching_dec_fx()

Affected operating points

• Non-BE for stereo switching cases in the sequence MDCT stereo -> DFT stereo -> TD stereo