README.android

# Android differences from upstream chromium

Android builds with `C_ARITH_CODING_SUPPORTED` and `D_ARITH_CODING_SUPPORTED`,
added in
https://android-review.googlesource.com/c/platform/external/libjpeg-turbo/+/291407/
to support a variant of jpeg files using arithmetic (instead of Huffman)
encoding. This variant isn't often used because of a lack of support in many
viewers (e.g. Chromium), but Android really values backwards compatibility, and
this might break some users. Android probably only needs to keep
`D_ARITH_CODING_SUPPORTED`, but vendor code might also be encoding by setting
jpeg_compress_struct.arith_code to true, so we enable both to ensure full
backwards compatibility since it's not really costing us anything.
We `#define` these in jconfig.h rather than in Android.bp so that they're
correctly exported to any *users* (in particular, jerror.h only conditionally
defines the corresponding error codes if these `#define`s are present).

Cherry-pick on 2024/01/08: https://github.com/libjpeg-turbo/libjpeg-turbo/commit/c27695a193a7cb265ef8e45c7b28bda3a71c70ce