Update and rename main.yml to flutter.yml

Author: cloudwebrtc

.github/workflows/flutter.yml

@@ -0,0 +1,23 @@
+name: Flutter action
+
+on: [push]
+
+jobs:
+  test:
+    name: Test on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-java@v1
+        with:
+          java-version: '12.x'
+      - uses: subosito/flutter-action@v1.1.1
+        with:
+          flutter-version: '1.8.4'
+          channel: 'beta'
+      - run: flutter pub get
+      - run: flutter test
+      - run: flutter build apk