Create main GitHub Actions workflow

.github/workflows/main.yml

@@ -0,0 +1,25 @@
+name: Build & Test
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: macOS-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install Java Development Kit 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Build APK & Run small tests
+      run: android/build.sh build
+    - name: Run medium tests
+      uses: ReactiveCircus/android-emulator-runner@v2.2.0
+      with:
+        api-level: 29
+        script: android/build.sh medium-tests
+    - name: Upload artifacts
+      uses: actions/upload-artifact@v1
+      with:
+        name: Build
+        path: android/uhabits-android/build/outputs/