chore: added in-app event listeners

[Shahroz16]

closes: MBL-664: Initialize In-App Module in Flutter SDK

changes:

• Added in-app listener
• Refactored plugins to be activity-aware
• Moved in-app listener to the in-app plugin

// Before:
CustomerIO.instance.subscribeToInAppEventListener(handleInAppEvent);
// After
CustomerIO.instance.inAppMessaging.subscribeToInAppEventListener(handleInAppEvent);

[github-actions]

Sample app builds 📱

Below you will find the list of the latest versions of the sample apps. It's recommended to always download the latest builds of the sample apps to accurately test the pull request.

---

• amiapp_flutter: 180.8.0 (28874339)

[mrehan27]

There are a few overlaps with some refactoring I'm working on in another ticket, so I might tweak those parts slightly later, rest looks good!