chore(datafile manager): Added async storage as dev dependency (#455)

Summary:

Datafile Manager declares @react-native-community/async-storage as a peer dependency. This is required to be in node_modules when typescript is transpiled before publishing or during development. Adding it as a dev dependency in addition to the peer dependency.

Test plan:

All the existing test pass

Author: zashraf1985

packages/datafile-manager/package-lock.json

@@ -39,7 +39,8 @@
     "nock": "^10.0.6",
     "prettier": "^1.19.1",
     "ts-jest": "^24.0.0",
-    "typescript": "3.3.3333"
+    "typescript": "3.3.3333",
+    "@react-native-community/async-storage": "^1.2.0"
   },
   "dependencies": {
     "@optimizely/js-sdk-logging": "^0.1.0",