avif: Use grid size to query for codec

When processing AVIF images with grid, use the size of each grid
to query for the MediaCodec instance rather than using the size of
the whole image.

CtsMediaMiscTestCases (which contains the AVIF tests continue to
pass).

Test: atest CtsMediaMiscTestCases:MediaMetadataRetrieverTest CtsMediaMiscTestCases:ThumbnailUtilsTest
Change-Id: Id783461066086c8ff9ea978d48c8525051e72e65