Android: Add a way to inform Qt user that Activity has changed

Especially in Qt Quick for Android, it's possible that the Activity changes during the lifetime of the Qt part of the app. The Activity/Context is also exposed to the user via a public API, https://doc.qt.io/qt-6/qnativeinterface-qandroidapplication.html#context

So, the user could either hold a reference to that Context in their C+/Qt code, or even pass it to some Android APIs, e.g. to register BroadcastReceivers. Also, Qt Android codebase internally uses this Context a lot, also in C+. So there should be some way, in the public API, to let the user know that the Activity/Context has changed, and any references to it should be cleared etc.