Migrate Android to SDL3

Use SDL3's official Android AAR through Prefab, update the native and
Java integration, and remove the bundled SDL2 Android sources.

.github/workflows/android.yml

@@ -4,6 +4,7 @@ on: [push, pull_request]
 jobs:
   build:
     runs-on: ubuntu-latest
+    name: Android SDL3
 
     steps:
     - uses: actions/checkout@v6
@@ -26,5 +27,6 @@ jobs:
     - name: Upload artifacts
       uses: actions/upload-artifact@v7
       with:
+        name: system3-android-sdl3
         path: android/app/build/outputs/apk/release/app-release.apk
         archive: false