Update the still AVIF parser to only read the primary item. With this
patch, AVIF still images with exif/icc/alpha channel will no longer
fail to parse.

For example, this patch enables parsing of files in:
https://github.com/AOMediaCodec/av1-avif/tree/master/testFiles/Microsoft

Adding two fate tests:
1) demuxing of still image with 1 item - this test will pass regardless
   of this patch.
2) demuxing of still image with 2 items - this test will fail without
   this patch and will pass with patch applied.

Partially fixes trac ticket #7621
Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: James Zern <jzern@google.com>