Publish Android JNI libraries as JVM client classifiers

[dvrdnz]

Summary

Add Android JNI libraries as classifier artifacts to the existing libsignal-client JVM publication.

The normal libsignal-client JAR remains unchanged and continues to provide the Java API. Android JNI libraries are published separately as architecture-specific classifier JARs containing only libsignal_jni.so.

Published artifacts

For each Android ABI, the JVM client publication additionally produces:

libsignal-client-<version>-android-aarch64.jar
libsignal-client-<version>-android-arm.jar
libsignal-client-<version>-android-x86_64.jar
libsignal-client-<version>-android-x86.jar

Each classifier JAR contains:

libsignal_jni.so

The classifier artifacts are therefore usable independently from the normal JVM artifact while avoiding changes to the existing desktop JNI packaging.

Implementation

The :client publication now:

1. depends on the existing Android JNI build;
2. collects the JNI libraries from the corresponding Android ABI directories;
3. creates one classifier JAR per Android ABI;
4. attaches those JARs to the existing mavenJava publication.

The existing desktop JNI artifacts and normal libsignal-client JAR are unchanged.

Validation

The implementation was verified with the complete Android JNI build:

aarch64-linux-android
armv7-linux-androideabi
x86_64-linux-android
i686-linux-android

The resulting local Maven publication contains:

libsignal-client-<version>.jar
libsignal-client-<version>-android-aarch64.jar
libsignal-client-<version>-android-arm.jar
libsignal-client-<version>-android-x86_64.jar
libsignal-client-<version>-android-x86.jar

The classifier JARs were inspected and each contains exactly:

META-INF/
META-INF/MANIFEST.MF
libsignal_jni.so

The publication was successfully built and published to Maven Local.

Consumer model

The intended consumer model is:

Desktop JVM
  → org.signal:libsignal-client:<version>

Android aarch64
  → org.signal:libsignal-client:<version>
  → org.signal:libsignal-client:<version>:android-aarch64

The normal artifact remains responsible for the Java classes; the classifier artifact supplies the platform-specific JNI library.

Notes

This change intentionally does not introduce Android platform detection or modify consumers. Consumer-side selection of the appropriate classifier is handled separately.

[jrose-signal]

What's the motivation here? What happens to the existing Android-specific JVM classes?

[dvrdnz]

The motivation is that "signal-cli" on Termux runs as a plain JVM app, but on Android it still needs the Android-built libsignal_jni.so. The existing Android JNI is packaged as part of libsignal-android, which isn't directly usable as a JVM-only dependency.

This PR doesn't change the Android packaging. It only adds the Android JNI .so files as additional classifier JARs to the existing libsignal-client publication. The normal libsignal-client JAR remains unchanged; each classifier JAR contains only libsignal_jni.so.

The existing Android-specific JVM classes aren't touched. They remain in the Android library/AAR, and :android continues to depend on :client as before.

So Android apps keep using libsignal-android as-is, while JVM consumers on Android (such as Termux) can use libsignal-client together with the matching Android JNI classifier.

I've also verified this end-to-end with signal-cli on Termux (AsamK/signal-cli#2106), including build, registration, verification, and successfully sending a message.