SystemUiVisibility overwritten bug (#17370)

Summary:
Make StatusBar style respect previously set systemUiVisibility

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I tried to set `SystemUiVisibility` flag to `SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR` to enable Android's light soft navigation bar, but when I had `<StatusBar />` component in my view, then it was always overwritten. This is how I found the bug and fixed it.

1. In MainActivity you can set systemUiFlags like this in onCreate method
```
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            Window window = getWindow();
            View rootView = window.getDecorView().getRootView();
            rootView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR);
        }
```
2. Add <StatusBar /> to a view
3. In `android/app/build.gradle` file set `targetSdkVersion` to **27** instead of 22 or something like that
4. In `android/app/src/main/res/values/styles.xml` add 2 lines to `AppTheme`
```
        <item name="android:windowDrawsSystemBarBackgrounds">true</item>
        <item name="android:navigationBarColor">#ffffff</item>
```
5. Run the app.
6. Test. Previously bottom soft navigation bar was set to white and buttons also to white (so they weren't visible anymore), because StatusBar was overwriting previously set systemUiVisibility Flags. Now the bar should be white and buttons dark as expected.

Previous buggy android bottom navbar
<img width="379" alt="screen shot 2017-12-27 at 17 11 57" src="https://user-images.githubusercontent.com/571171/34385126-2a3edc44-eb29-11e7-812e-846cfd2fb88b.png">

New fixed android bottom navbar
<img width="379" alt="screen shot 2017-12-27 at 17 12 07" src="https://user-images.githubusercontent.com/571171/34385137-3e82517c-eb29-11e7-8af9-a3b7a41ae8e2.png">

This pull request does not change any existing feature. It only makes StatusBar coloring work more properly without affecting bottom navigation bar on android or any other system ui visibility feature.

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[ANDROID] [BUGFIX] [StatusBar] - Fixed StatusBar overwriting previously set SystemUiVisibility flags
Pull Request resolved: https://github.com/facebook/react-native/pull/17370

Differential Revision: D13860079

Pulled By: cpojer

fbshipit-source-id: a0bca7acb7601eb78f0842239ea4dee76a63d1fd

ReactAndroid/src/main/java/com/facebook/react/modules/statusbar/StatusBarModule.java

@@ -189,8 +189,13 @@ public class StatusBarModule extends ReactContextBaseJavaModule {
           @Override
           public void run() {
             View decorView = activity.getWindow().getDecorView();
-            decorView.setSystemUiVisibility(
-              "dark-content".equals(style) ? View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR : 0);
+            int systemUiVisibilityFlags = decorView.getSystemUiVisibility();
+            if ("dark-content".equals(style)) {
+              systemUiVisibilityFlags |= View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR;
+            } else {
+              systemUiVisibilityFlags &= ~View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR;
+            }
+            decorView.setSystemUiVisibility(systemUiVisibilityFlags);
           }
         }
       );