@Will: switched to the Back4App as backend support

Switched to back4app for backend support in order to bypass GFW in commit https://github.com/RichardeJiang/from_android/commit/07c2103a2181716f97d99136c18289ce4bafbad8. You'll need the secret constant file (place it in the /lib dir). I've uploaded the file to the shared Google Drive folder.