How to exclude a file from an .AAR Android library archive with gradle

I am trying to create an AAR file for a library Android project using Android Studio and gradle.

I want to exclude from this archive specific folders and files but I cannot find a working solution.

The project has 2 flavours.

app/
|--libs/
|--src/
   |--flavour1/
   |  |--java/
   |     |--a1/
   |     |  |--class_File1.java
   |--flavour2/
   |  |--java/
   |     |--a1/
   |     |  |--class_File1.java
   |--main/
      |--java/
      |  |--...
      |--res/
      |  |--raw/
      |  |  |--comments.txt
      |  |--...
      |--AndroidManifest.xml

and I use a build.gradle file like this one

apply plugin: 'com.android.library'

android {
compileSdkVersion 21
buildToolsVersion "21.1.2"

defaultConfig {
    minSdkVersion 10
    targetSdkVersion 21
}
packagingOptions {
    exclude 'assets'
    exclude '**/comments.txt'
}


sourceSets {

        flavour1 {
            resources {
                exclude '**/comments.txt'
            }
        }
        flavour2 {

        }

}

buildTypes {
    release {
        minifyEnabled true
        shrinkResources true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
    }
}

productFlavors {

    flavour1 {

       packagingOptions {
            exclude 'assets'
            exclude 'res/raw/comments.txt'
        }
    }
    flavour2 {
    }
}
}

dependencies {
compile 'com.google.android.gms:play-services:+'
}

Generally speaking, I have experimented a lot. I have tried packagingOptions, exclusion in sourceSets but nothing seems to work. What I am trying to achieve is not include for example in the .AAR archive the file comments.txt and the folder "assets" which is an optional folder. I examine each time the .AAR file by renaming it to zip and looking into the files included.

I have also looked into the documentation here, where maybe configurations could be a solution but I am not sure how to use it or how to move forward. Any help will be appreciated.