RN: Simplify `DevSettings` Implementation
RN: Simplify `DevSettings` Implementation

Summary:
Simplifies the implementation of `DevSettings`. The fact that it uses `NativeEventEmitter` is an implementation detail and does not need to be exposed via inheritance.

This also enables more code to be removed from production builds (because the development implementation is now statically enclosed by `__DEV__`).

Changelog:
[General][Changed] - `DevSettings` no longer inherits from `NativeEventEmitter`

Reviewed By: RSNara

Differential Revision: D26140148

fbshipit-source-id: 99fa9d0c6ce8e365f89936aa12a4720f7a04b984

facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,pandiaraj44/react-native,janicduplessis/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,myntra/react-native,pandiaraj44/react-native,myntra/react-native,facebook/react-native,facebook/react-native,pandiaraj44/react-native,myntra/react-native,pandiaraj44/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,myntra/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,myntra/react-native,pandiaraj44/react-native,javache/react-native,arthuralee/react-native,arthuralee/react-native,arthuralee/react-native,javache/react-native,arthuralee/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,janicduplessis/react-native,pandiaraj44/react-native,myntra/react-native