(demo): Set more precise React/React-Native package version dependencies for Example (#679)

* Set more precise React/React-Native package version dependencies

- In general, try to avoid breaking changes caused by >=
  - In particular, such a breaking change can be seen by testing the current Example
    app on Android with the existing package.json's and RN 0.43+, which causes errors
    in MainApplication public/protected method settings.

* Revert changes to main package.json

- Would erroneously break compatibility for main package
- Leave precise dependencies only for Example app, since this does not impact
  compatibility in the same way

Example/package.json

@@ -6,8 +6,8 @@
     "start": "node node_modules/react-native/local-cli/cli.js start"
   },
   "dependencies": {
-    "react": ">=15.4.0",
-    "react-native": ">=0.40",
+    "react": "~15.4.0",
+    "react-native": "~0.40.0",
     "react-native-camera": "file:../"
   }
 }