gradle: package *.jar into aar

build.gradle

apply plugin: 'com.android.library'

android {
    compileSdkVersion 28

    defaultConfig {
        minSdkVersion 14
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

configurations {
    includeJars // define new configuration
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.google.android.gms:play-services:12.0.1'
    includeJars(name:'third_party_legacy_do_not_touch', ext:'jar') // the needed JAR(s) is/are here
    testImplementation 'junit:junit:4.12'
}

// Main "magic" script to do the job
project.afterEvaluate {
    def isAndroidLibraryProject = project.plugins.hasPlugin('com.android.library')
    if(isAndroidLibraryProject) {
        task copyDeps(type:Copy) {
            from configurations.includeJars {
                include '**/*.jar'
            }
            into "./build/intermediates/packaged-classes/release/libs" // this folder gets packaged inside the AAR
        }
        mergeReleaseJniLibFolders.dependsOn copyDeps // only this stage worked for me - neither earlier, nor later

        task copyDebugDeps(type:Copy) {
            from configurations.includeJars {
                include '**/*.jar'
            }
            into "./build/intermediates/packaged-classes/debug/libs"
        }
        mergeDebugJniLibFolders.dependsOn copyDebugDeps
    }
}