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

[x] Fix for 3gpp issue: #825 (closed) (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.

Pipeline results:

• EVS 26.444 BE test passes.
• decoder pytests:
  MLD scores:
  Original inputs: 562 failed, 374 skipped
  MLD > 10: 12
  MLD > 5 : 76
  MLD < 5 : 474
  +10dB inputs: 562 failed, 374 skipped
  MLD > 10: 107
  MLD > 5 : 180
  MLD < 5 : 275
  -10dB inputs: 562 failed, 374 skipped
  MLD > 10: 2
  MLD > 5 : 21
  MLD < 5 : 539
• encoder pytests results:
  MLD scores:
  Original inputs: 562 tests:
  MLD > 10: 18
  MLD > 5 : 71
  MLD < 5 : 473
  +10dB inputs: 562 tests:
  MLD > 10: 66
  MLD > 5 : 124
  MLD < 5 : 372
  -10dB inputs: 562 tests:
  MLD > 10: 5
  MLD > 5 : 30
  MLD < 5 : 527