Two functions in fd_cng_dec converted to fxd

Added float to fixed changes to the following functions:
[x] FdCngDecodeMDCTStereoSID
[x] FdCngDecodeDiracMDCTStereoSID
[x] dtx_read_padding_bits
[x] Made some changes to msvq_dec_fx for ivas specific code.