The current code has two issues with this combination:
i) The frame_size_override_flag was being set based on the downscaled
   frame size rather than the upscaled frame size
ii) This flag was then being ignored in write_frame_size(), and we
    would always write a 16-bit size, even if the flag was 0.
    This lead to an encode/decode mismatch.

This patch fixes these two issues, and rewrites write_frame_size()
to be much clearer and more closely match setup_frame_size() from
the decoder.

Change-Id: Id6b8e51ef10f3285f8318e4acd8606060f29e9cb