Change internal sampling rate for ISM at 48 kbps

Currently, the ISM format at 48 kbps for 1ISM uses as a core-coder switched ACELP/TCX model at an internal sampling rate of 16 kHz.

On the other hand, EVS at the same bitrate benefits from a TCX-only core-coder at an internal sampling rate of 25.6 kHz.

It is thus proposed to change the internal sampling of ISM format to match that of EVS at 48 kbps. This also means that the maximum total_brate where ACELP is supported is lowered down from 48 kbps to 40 kbps in the ISM format. Other formats will not be affected.

Note: the same issue exists at 96 kbps for 2 ISM, or at 192 kbps for 4 ISM.