Redirecting users to battery optimisation screen

Suggestion: Since the wiki's best practices recommend disabling battery optimisation, it would be easy if users would be redirected to this screen directly when starting a study (if it's not disabled already).

Per this [SO post](https://stackoverflow.com/questions/41596509/android-m-startactivity-battery-optimization), redirecting users to the battery optimisation screen (it cannot be done directly) can be achieved by adding the `Settings.ACTION_IGNORE_BATTERY_OPTIMIZATION_SETTINGS` flag to an intent. Alternatively, `Permission.ignoreBatteryOptimizations.request()` of `flutter-permission-handler` is meant to do the same.