[Non-BE] Resolve "Two decoding paths in the decoder"

• Related issues: #857 (closed), #500 (closed), #507 (closed)
• Requested reviewers:

Reason why this change is needed

The decoder has two path, whether JBM or 5ms decoding is active either through the JBM path or through the 20ms path. This doubles a lot of functionality and make the maintenance of the code harder since all fixes need to be checked if they are present in both paths.

Description of the change

Unify decoding paths. Streamline API with respect to allowed render frame sizes.

Affected operating points

• Discrete ISM to BINAURAL_ROOM_IR with head rotation modes are slightly different due to necessary different usages of the head positions.
• Produced bitstream is BE compared to previous state (no encoder changes).