Draft: Resolve "Simplification of arithmetic codec subfunctions"

Harmonisation of the arithmetic decoder functions.

(encoder functions are handled in MR 2782)

ari_start_decoding_14bits_fx() / ivas_ari_start_decoding_14bits_ext_1_lfe()

ari_decode_14bits_bit_ext_fx() / ivas_ari_decode_14bits_bit_ext_1_lfe()

Closes #2402