Fix for issue 771: Frequency peak for DTX Stereo at lower bitrates and issue 772: Noise Burst for DTX Stereo at lower bitrates with Fs 16kHz

EVS 26.444 BE test passes.
pytests results: 597 failed, 381 skipped

MLD scores:
Original inputs:
MLD > 10: 57
MLD > 5 : 122
MLD < 5 : 418

+10dB inputs:
MLD > 10: 188
MLD > 5 : 196
MLD < 5 : 213

-10dB inputs:
MLD > 10: 19
MLD > 5 : 60
MLD < 5 : 518