fix: don't replace existing MediaQuery widget

:scroll: Description

Before wrapping the app widget in a new MediaQuery it's checking for an existing one. If there's already a `MediaQuery' Widget, it doesn't replace the old one.

:bulb: Motivation and Context

I don't add the BetterFeedback widget above the MaterialApp, instead in the builder from MaterialApp, because I'm using DevicePreview and ResponsiveFramework. They have to be above the BetterFeedback widget and manipulate the MediaQuery. So I changed this package to use the existing MediaQuery widget, if any.

:green_heart: How did you test it?

I have tested the changes manually in my own project.

:pencil: Checklist

[x] I reviewed submitted code
[ ] I added tests to verify changes
[x] I updated the docs if needed
[x] All tests passing
[x] No breaking changes

ueman / feedback