Allow setting prefix in shared preference - for migrating from old app that did not use flutter #52544
Description
Use case
I have an app (based on Xamarin, but I that's not importing) that I am migrating to flutter.
The App uses share preferences (NSUserDefaults (on iOS) and SharedPreferences (on Android)) to store some user configuration data.
It would be very nice, if these settings are not lost when migrating to flutter.
But they are (at least in my tests), and I think that is because flutter uses the 'flutter.' prefix for any preferences one stores.
Proposal
Add an option to the shared preference plugin to change the ".flutter" prefix to something else (or even make it empty).
This could be simply done by making the _prefix memeber non-const and adding a static function to change it.