Add Flutter Build Apk workflow

.github/workflows/flutter_build_apk.yml

@@ -0,0 +1,26 @@
+name: Flutter Build Apk
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-java@v1
+        with:
+          java-version: "12.x"
+      - uses: subosito/flutter-action@v1
+        with:
+          channel: "beta"
+      - run: flutter pub get
+      - run: flutter format --set-exit-if-changed .
+      - run: flutter analyze .
+      - run: flutter build apk
+      - uses: actions/upload-artifact@v1
+        with:
+          name: release-apk
+          path: build/app/outputs/apk/release/app-release.apk