Add GitHub Actions workflow for continuous integration.

- Create `android.yml` workflow to automate builds on pushes and pull requests to `main` and `master` branches.
- Configure build environment with JDK 17 (Temurin distribution) and Gradle caching.
- Implement build step to generate the debug APK using `./gradlew assembleDebug`.
- Add artifact upload step to preserve the resulting `app-debug.apk` for each successful build.

Author: robinji0

.github/workflows/android.yml

@@ -0,0 +1,33 @@
+name: Android CI
+
+on:
+  push:
+    branches: [ "main", "master" ]
+  pull_request:
+    branches: [ "main", "master" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: set up JDK 17
+      uses: actions/setup-java@v4
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+        cache: gradle
+
+    - name: Grant execute permission for gradlew
+      run: chmod +x gradlew
+
+    - name: Build with Gradle
+      run: ./gradlew assembleDebug
+
+    - name: Upload APK
+      uses: actions/upload-artifact@v4
+      with:
+        name: app-debug
+        path: app/build/outputs/apk/debug/app-debug.apk