build(repo): gitignore Android/Gradle build artifacts

Adds .gradle/, local.properties, and **/jniLibs/**/*.so to the
ignore list. The jniLibs .so files (600 KB - 950 KB per ABI) are
produced by the cargoBuildRust Gradle task wrapping cargo-ndk;
they are regenerated on every build and must not be tracked.

Required by the next commit (the Android audio spike) which
otherwise would try to track those four prebuilt libraries.

.gitignore

@@ -79,5 +79,13 @@ chanora-docs-repo-format-*.zip
 # FRB / cargokit local build dirs
 **/cargokit_build/
 
+# Android / Gradle local state
+**/.gradle/
+**/local.properties
+
+# JNI native libraries produced by cargoBuildRust / cargo-ndk
+# (regenerated by the Gradle build; not project source).
+**/jniLibs/**/*.so
+
 # IntelliJ / Android Studio per-project
 **/*.iml