movdec: Set frame_size for AMR

Earlier, sc->samples_per_frame was used for setting the frame size,
but all files don't have that set properly. The frame size is a
known constant for these codecs.

If frame_size isn't set, the mov/3gp muxer refuses to mux it.

This fixes stream copy of audio from
https://roundup.libav.org/file1248/Video_With_AMR-NB_Audio.3gp


to another 3gp file (roundup issue 2468).

Signed-off-by: Martin Storsjö <martin@martin.st>