Modify tasks (afterEvaluate) in a single flavor Gradle

Currently I have two flavors in my gradle build: googleplay and amazon.

I'm using the multidex gradle solution and amazon requires to add the following task to the gradle build in order to save methods and apply their DRM:

android{
  ....
}


afterEvaluate {
    tasks.matching {
        it.name.startsWith('dex')
    }.each { dx ->
        if (dx.additionalParameters == null) {
            dx.additionalParameters = []
        }
        dx.additionalParameters += "--set-max-idx-number=55000"

    }
}

How can I make that code only apply to the amazon flavor?

Thanks