movenc: enable NAL reformatting for HEVC tracks.

Also, use the same check as ff_isom_write_avcc() to
determine whether the bitstream is in Annex B format.
matroskaenc: enable NAL reformatting for HEVC tracks.

Also, use the same check as ff_isom_write_avcc() to
determine whether the bitstream is in Annex B format.