Exoplayer: Use okhttp version specified in gradle.properties (#2340)

Because React Native uses okhttp, including exoplayer causes apps to use two different versions of okhttp.  This results in some unpredictable behavior.  Clients of `react-native-video` should be able to specify the same OKHTTP version to react-native and react-native video.

See where it's specified in react-native trunk: 
- https://github.com/facebook/react-native/blob/master/ReactAndroid/gradle.properties#L15
- https://github.com/facebook/react-native/blob/e1b6cd3f756aa034b11af6bf9960efb42bde8692/ReactAndroid/build.gradle#L452-L453