Fix foreground service start not allowed exception (resolves bitfireAT/davx5#32)

* Make "Battery optimization whitelisting" a global DAVx5 setting
* Make "Keep in foreground" setting dependent of "Battery optimization whitelisting" setting
* if foreground service is enabled, remind user to enable battery optimization whitelisting too, before starting the foreground service

Co-authored-by: Ricki Hirner <hirner@bitfire.at>