handleActivation

Registers a callback to handle all notification activations. The callback is invoked whenever a notification is clicked, replied to, or has an action button pressed - regardless of whether the original Notification object is still in memory.

This method handles timing automatically:

• If an activation already occurred before calling this method, the callback is invoked immediately with those details.

• For all subsequent activations, the callback is invoked when they occur.

The callback remains registered until replaced by another call to handleActivation.

This provides a centralized way to handle notification interactions that works in all scenarios:

• Cold start (app launched from notification click)

• Notifications persisted in AC that have no in-memory representation after app re-start

• Notification object was garbage collected

• Notification object is still in memory (callback is invoked in addition to instance events)