Initial setup for CircleCI (#257)

Summary:
This is so much faster! This should allow us to more easily publish snapshots.
Pull Request resolved: https://github.com/facebook/flipper/pull/257

Reviewed By: danielbuechele

Differential Revision: D9630067

Pulled By: passy

fbshipit-source-id: 64c633933a971cc15371d2ee90c184e13b98e06a

.circleci/config.yml

@@ -0,0 +1,14 @@
+version: 2.0
+jobs:
+  build:
+    docker:
+      - image: circleci/android:api-28-ndk-r17b
+    steps:
+      - checkout
+      - run: ./gradlew :sample:assembleDebug
+
+workflows:
+  version: 2
+  build:
+    jobs:
+      - build