Changes

trau_frame: fix de/encoding of TRAU-AMR-8k-6k7 format

There exist 3 TRAU frame types (distinguished by sync pattern) for
AMR on 8 kbit/s subslots: Low, 6k7 and 7k4.  osmo_trau_frame_encode()
and osmo_trau_frame_decode_8k() APIs support all 3 TRAU-AMR-8k frame
types since the original 2020 implementation - however, encoding and
decoding of 6k7 format was incorrect.  Fix it.

Correctness of the new implementation is verified by unit tests
added with the next patch in this series.

Change-Id: Ibb98b0c189ced5248dca217384ca34b6a5f8b210