Build Android ffmpeg AAR with x264 and 16KB validation

[neeraj-pilot]

Summary

• build the Android AAR from CI using x264, zimg, Android zlib, and GPL flags
• validate the generated AAR for ARM ABIs, x264/zimg/zlib/GPL, no packaged libc++_shared.so, CI build paths, and 16 KB LOAD alignment
• replace the vendored Flutter Android AAR with the CI-built artifact

Validation

• GitHub Actions run: https://github.com/neeraj-pilot/ffmpeg-kit/actions/runs/26016716521
• Downloaded artifact SHA-256: d37620efe2d949e9c30f73893c8cd6476aa87e01581058c651f6724a94c7bc56
• Artifact size: 23,565,018 bytes
• Local validation against vendored AAR: scripts/validate-android-aar.sh passed with NDK 26.3.11579264 llvm-readelf
• Embedded FFmpeg config path confirms remote build: /home/runner/work/ffmpeg-kit/ffmpeg-kit/.ndk/android-ndk-r26d/...