EVS: Support decoding Primary 64.0 kbps packets

From "7.1.5 Bit allocation at 48, 64, 96 and 128 kbps " in https://www.etsi.org/deliver/etsi_ts/126400_126499/126445/16.01.01_60/ts_126445v160101p.pdf: We can see there is 1 TCX/HQ-MDCRT core flag bit for 64 kbps. But I counldn't find the description of this flag in document:

At 64 kbps, the information about using the CELP core or the HQ-MDCT core is transmitted as a 1-bit CELP/MDCT core flag. If CELP core is selected, the BW and CT parameters are combined together to form a single index. In the case of HQ-MDCT core, the next one or two bits signal whether the previous frame was encoded with the CELP core or not. The second bit is used to signal its internal sampling rate (12.8 or 16 kHz) only when the previous frame was encoded with the CELP core.

Besides that, since 64 kbps doesn't support TCX, it seems weird to have this TCX/HQ-MDCRT core flag. So in this PR, I assume there is no TCX/HQ-MDCRT core flag.

I am not an expert in EVS, so need the reviewers to double-check, thanks very much in advance!