In App Messaging Pause Display

It would be great to be able to disable and enable showing an in app message until the app is initialized, currently it is showing up on our splash screen then being dismissed as the main activity starts.

Iterable provided the ability to disable and enable the display of the in app messages through the singleton. This also was valuable so we could block an in app message from popping up during a critical flow in the app.

A proposed API would look like

```
CustomerIO.instance().inAppMessaging().disable()
CustomerIO.instance().inAppMessaging().enable()
```

Start by locating the CustomerIO.instance().inAppMessaging() singleton and the existing in-app message display lifecycle. The issue names no files or tests, so map how messages appear during splash-screen initialization and critical flows. Done means disable prevents display until enable is called, while normal display resumes afterward.

Written by the indexing model from the issue text.