Step 1: First of all click "Add/Edit Configurations" as like image 1:
Step 2: Choose "Flutter" from configuration list and select your Dart entrypoint path according to your flavor then give a name and allow dart support for this project. See on Image 2, 3 and 4:
Step 3: Create another configuration for prod following two steps above:
N.B: In this step you may need to setup your dart sdk path. If you get warned for "dart sdk not found in specified location" then just click on "fix" and select your dart sdk path.
Release build can be made using flutter build <options> apk command. Before that create a key.properties file in the android folder. Add folloiwng lines to that file:
keyAlias=<your keystore alias>
password=<your keystore password>
storeFile=<path to the keystore file>
Then run flutter build appbundle -t lib/main_prod.dart --flavor prod to generate app bundle (.aab) file. To generate .apk file, run flutter build apk -t lib/main_prod.dart --flavor prod.
For more details, please read through guide to build and relase android app documentation from this link.