Update NotificationChannelManager with LanguageContext instance

* Update NotificationChannelManager createChannel to use LanguageContext instance
* Rename deviceLanguage variable to language for consistency

Author: tanaynigam

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

@@ -115,9 +115,9 @@ private static String createChannel(Context context, NotificationManager notific
       JSONObject payloadWithText = channelPayload;
       if (channelPayload.has("langs")) {
          JSONObject langList = channelPayload.getJSONObject("langs");
-         String deviceLanguage = OneSignal.languageContext.getLanguage();
-         if (langList.has(deviceLanguage))
-            payloadWithText = langList.optJSONObject(deviceLanguage);
+         String language = LanguageContext.getInstance().getLanguage();
+         if (langList.has(language))
+            payloadWithText = langList.optJSONObject(language);
       }
 
       String channel_name = payloadWithText.optString("nm", "Miscellaneous");