Exercise subset decoding in FuzzAndroidCodec

Extend FuzzAndroidCodec to test getSupportedSubset(),
getSampledSubsetDimensions(), and getAndroidPixels() with fSubset
alongside fSampleSize so that subset-specific bugs (such as the OOB
write fixed in b/562438701) are covered by fuzzing.

Bug: b/562799124
Change-Id: I13837da3c2f27c5e3e68ff1aa0ce055fdf99e8ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1379817
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>