Add Android cross-compilation to CI and publish .so artifacts

[nbassler]

Add Android cross-compilation to CI and publish .so artifacts

Currently the dEdx Android app vendors a copy of the
libdedx C source directly in the repository and compiles it as part of the Android NDK build.
This is fragile: the vendored copy drifts out of sync, and any changes to libdedx require a
manual sync.

The clean solution is to have libdedx publish pre-built .so files for Android ABIs as GitHub
release artifacts, so the app can download and embed them directly without building from source.

What is needed

Add a GitHub Actions workflow to libdedx that cross-compiles for the following Android ABIs
using the Android NDK:

• arm64-v8a (primary target, modern devices)
• armeabi-v7a (32-bit ARM, older devices)
• x86_64 (emulator)

And publishes the resulting libdedx.so files as artifacts on each release.

Expected outcome

The dEdx Android app build can then:

1. Download the pre-built .so files for each ABI
2. Place them in app/src/main/jniLibs/<abi>/libdedx.so
3. Remove the vendored libdedx/ source tree entirely
4. Reduce CMakeLists.txt to just compiling the thin JNI wrapper dEdx.c against the
   pre-built library

This gives a clean separation between libdedx development and app development, and ensures
the app always uses an officially built and tested version of the library.