Currently we are differentiating between a FlutterActivity and a FlutterFragmentActivity with instanceof. We should find a better way to differentiate, or more generally to get the flutter view in FlutterDeviceScreenshot.

Context
https://github.com/flutter/flutter/pull/132406/files#r1313387041
go/UnhealthyIdioms#awkward-inheritance-hierarchy