Standalone AAR build script

[GregoryComer]

🚀 The feature, motivation and pitch

We currently provide pre-built Android AARs for download. It would be desirable to have a standalone build script (maybe build_android_libraries.sh?) that can reproduce the binaries with one line. The use case is for users to be able to easily switch to building from source if they need to change the configuration relative to the published binaries (adding backends/extensions, changing compiler flags, etc.). The current AAR build logic is under build/build_android_library.sh. We can likely split it out into a dedicate script.

Alternatives

No response

Additional context

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RFC (Optional)

No response

cc @kirklandsign @mergennachin @byjlw

[GregoryComer]

#8747 renames the existing script to build_android_library.sh. We can document the usage under using-executorch-building-from-source.md and using-executorch-android.md.