isGroupConversation
Returns true if this notification represents a group conversation, otherwise false.
If the application that generated this MessagingStyle targets an SDK version less than P and setGroupConversation was not called, this method becomes dependent on whether or not the conversation title is set; returning true if the conversation title is a non-null value, or false otherwise. This is to maintain backwards compatibility. Regardless, setGroupConversation has precedence over this legacy behavior. From P forward, setConversationTitle has no affect on group conversation status.