[workflow] add GHA configuration

First try to build app with GH Actions.

.github/workflows/android.yml

@@ -0,0 +1,22 @@
+name: Android CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/upload-artifact@v2
+      with:
+        path: app/build/outputs/apk/debug/*.apk
+    - name: set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Setup Android SDK
+      uses: android-actions/setup-android@v2
+
+    - name: Build with Gradle
+      run: ./gradlew assembleDebug
+