Ask for battery excemption using dialog

[MatsG23]

This makes the app ask for a battery excemption in the introduction using a dialog the user can allow/decline in the app. The Android Project usually forbids the use of the needed permission for it. But

1. we already use it (I think you added it for flutter_background)
2. AFAIK, it is not planned to release the app to the Play Store
3. the use case of this app should allow the usage because it is "safety app" - Acceptable use cases for excemption

[MatsG23]

Before this may be merged, I want to clean up and add a few more small changes.

[MatsG23]

Done!