DTX for unified stereo @ 32000bps

In EVS when dtx is enabled there is an automatic switching between the two types of comfort noise, LP-CNG and FD-CNG. The same switching is used for IVAS unified stereo. In EVS, dtx is only allowed at high SNR (i.e. almost silent background) for 32000 bps at and then for some reason it is forced to use LP-CNG only. For IVAS there is no SNR condition at 32000 bps but the cng type is still forced to LP-CNG. This means that LP-CNG willl be used for background noise types where FD-CNG would be used for the lower bitrates. Assuming that the automatic switching of CNG type is beneficial there is no obvious reason for not allowing it for 32000 bps as well.

The fix has no impact on memory or max complexity. It is only a change of one condition in dtx.c

A fix is available in a a branch.