showMessageCenter  
  Called to show the message center for a specific message.
To show the message center, the SDK will try the following:
- The optional OnShowMessageCenterListener. 
- An implicit intent with - com.urbanairship.VIEW_RICH_PUSH_INBOX.
- If the message ID is provided, an implicit intent with - com.urbanairship.VIEW_MESSAGE_INTENT_ACTION.
- Finally it will fallback to the provided com.urbanairship.messagecenter.ui.MessageCenterActivity. 
Implicit intents will have the message ID encoded as the intent's data with the format message:<MESSAGE_ID>.
Parameters
messageId 
An optional message ID to display. If null, the inbox will be displayed.