Build Android sample app with GitHub Actions (#810)

Summary:
Getting really pissed off with Travis and this has significantly shorter queue times.

Mind that this probably won't work until the Folly PR is merged.
Pull Request resolved: https://github.com/facebook/flipper/pull/810

Test Plan: CI

Reviewed By: mweststrate

Differential Revision: D19951126

Pulled By: passy

fbshipit-source-id: ed51ad5ec6d4df3a59784dbd31e7c1667593d04f

.github/workflows/android-sample.yml

@@ -0,0 +1,22 @@
+name: Build Sample App
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Build with Gradle
+      run: ./gradlew :sample:assembleDebug
+    - name: upload artifact
+      uses: actions/upload-artifact@v1
+      with:
+        name: sample-app.apk
+        path: dist/Flipper-mac.zip