Returns the current context in which your game is running. This can be either
null or one of:
POST - The game is running inside of a Facebook post.
THREAD - The game is running inside a Facebook Messenger thread.
GROUP - The game is running inside a Facebook Group.
SOLO - This is the default context, the player is the only participant.
It is only populated if
contextGetType is in the list of supported APIs.
The context type.