Expose UI_MODE_TYPE_VR_HEADSET in PlatformConstants

Summary:
The Android version of PlatformConstants exposes the device's [UI Mode]( https://developer.android.com/guide/topics/resources/providing-resources#UiModeQualifier). It is missing the case for 'vrheadset', added in Android API 26. We should return the expected result when this is queried.

Changelog:
[Android][Added] - Expose UI_MODE_TYPE_VR_HEADSET in PlatformConstants

Reviewed By: rshest

Differential Revision: D38495875

fbshipit-source-id: fd904bd11213448415b7d75145d9ba6311ed407b

ReactAndroid/src/main/java/com/facebook/react/modules/systeminfo/AndroidInfoModule.java

@@ -50,6 +50,8 @@ private String uiMode() {
         return "desk";
       case Configuration.UI_MODE_TYPE_WATCH:
         return "watch";
+      case Configuration.UI_MODE_TYPE_VR_HEADSET:
+        return "vrheadset";
       case Configuration.UI_MODE_TYPE_NORMAL:
         return "normal";
       default: