StatusBar: barStyle support for Android (API 23+)

Summary:
Android (starting from API 23) supports "light status bar", thus it is possible to extend StatusBar and make `barStyle` property work not only for iOS, but also for Android.

This PR introduces one more `barStyle` option `dark-content` in addition to two existing ones (`default` and `light-content`).
Why there are 3 options instead of 2?

Two simple reasons:
1) to make all existing applications fully compatible with these changes;
2) the default status bar on Android is dark with white text and icons, while on iOS it is light with black text and icons on it. Thus the `default` option means something like "I don't really care, just apply the default color for this platform", while two other options (`light-content` and `dark-content`) allow to accurately specify the required result.
Closes https://github.com/facebook/react-native/pull/10185

Differential Revision: D3952346

fbshipit-source-id: 999a67614abff52321fbeb06298ebf1946c3f1d1

React/Modules/RCTStatusBarManager.m

@@ -18,6 +18,7 @@
 RCT_ENUM_CONVERTER(UIStatusBarStyle, (@{
   @"default": @(UIStatusBarStyleDefault),
   @"light-content": @(UIStatusBarStyleLightContent),
+  @"dark-content": @(UIStatusBarStyleDefault),
 }), UIStatusBarStyleDefault, integerValue);
 
 RCT_ENUM_CONVERTER(UIStatusBarAnimation, (@{