Fix for 3GPP issue 825: Missing synthesis when switching to CNG (24.4kbps)

[x] Fix for 3gpp issue: #825 (MLD reduced from 38.137897 to 9.840941)
[x] Removed all of the manual resets done with CNG related buffers and taking care of required scaling in ApplyFdCng_ivas_fx and stereo_dft_generate_comfort_noise_fx stacks.