Changing the initial route property of the navigation channel
after the initial execution of runApp() has no effect.
Developers who would like to use the same FlutterEngine
between different Activitys and Fragments and switch
the route between those displays need to setup a method channel and
explicitly instruct their Dart code to change Navigator routes.

Add a translucent Flutter screen

Most full-screen Flutter experiences are opaque.
However, some apps would like to deploy a Flutter
screen that looks like a modal, for example,
a dialog or bottom sheet. Flutter supports translucent
FlutterActivitys out of the box.

To make your FlutterActivity translucent,
make the following changes to the regular process of
creating and launching a FlutterActivity.

Step 1: Use a theme with translucency

Android requires a special theme property for Activitys that render
with a translucent background. Create or update an Android theme with the
following property:

<style