Convert OneSignal preferences and languageContext to private

* Convert OneSignal preferences and OneSignal languageContext from public to private since they are being called through dependency injection

Author: tanaynigam

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java

@@ -419,7 +419,7 @@ static Activity getCurrentActivity() {
    private static String userId = null, emailId = null;
    private static int subscribableStatus;
 
-   public static LanguageContext languageContext = null;
+   private static LanguageContext languageContext = null;
 
    // Is the init() of OneSignal SDK finished yet
    private static boolean initDone;