Android - ReactNativeHost getUseDeveloperSupport to public

Summary:
Currently React Native is opinionated in that the easiest approach is to extend ReactActivity. However to more easily allow integrating with existing application, we should allow some of the methods in ReactNativeHost to be public, and this is a very good first step.
* There is no harm in making this public from what I can tell.
* This allows `ReactNativeHost` to be more easily used outside of the `ReactActivity` and `ReactActivityDelegate` ecosystem. (A `ReactFragment` would be a good example)

_No issues found_

**Test plan (required)**

* Run any sample app and verify it still works.

Make sure tests pass on both Travis and Circle CI.
Closes https://github.com/facebook/react-native/pull/11329

Differential Revision: D4287429

Pulled By: AaaChiuuu

fbshipit-source-id: 8cb76f3226aae3737af5f5bd6010d3eea8df9bfe

ReactAndroid/src/main/java/com/facebook/react/ReactNativeHost.java

@@ -138,7 +138,7 @@ public abstract class ReactNativeHost {
   /**
    * Returns whether dev mode should be enabled. This enables e.g. the dev menu.
    */
-  protected abstract boolean getUseDeveloperSupport();
+  public abstract boolean getUseDeveloperSupport();
 
   /**
    * Returns a list of {@link ReactPackage} used by the app.