Add android sample apps to new releases (#615)

Summary:
Now on every new release, a sample app will be built and added to the Github release artifacts.

Adds a Github action to automatically build and attach the android sample app for every flipper release.
This will allow potential users to try out flipper without having to go to the trouble of building their own app.
Pull Request resolved: https://github.com/facebook/flipper/pull/615

Test Plan:
Tested on my private repo:
https://github.com/jknoxville/flipper/releases/tag/v0.1.john

Reviewed By: passy

Differential Revision: D18224747

Pulled By: jknoxville

fbshipit-source-id: 25d2905bd7539faa91974b23a33678168aef29d8

.github/workflows/android.yml

@@ -0,0 +1,27 @@
+name: Publish Sample App
+
+on: 
+  release:
+    types: [created]
+
+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: Rename apk
+      run: mv android/sample/build/outputs/apk/debug/sample-debug.apk SampleApp-android.apk
+    - name: Upload
+      uses: skx/github-action-publish-binaries@master
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        args: 'SampleApp-android.apk'