Please follow below steps to get it working ( I have tested it up to Android Studio 2.2)

Lets say you have kept aar file in libs folder. ( assume file name is cards.aar )

then in app build.gradle specify following and click sync project with Gradle files.
Open Project level build.gradle and add flatDir{dirs 'libs'} like did below

allprojects {
   repositories {
      flatDir {
        dirs 'libs'

and now open app level build.grdle file and add .aar file

    dependencies {
       implementation(name:'cards', ext:'aar')

If everything goes well you will see library entry is made in build -> exploded-aar

Also note that if you are importing a .aar file from another project that has dependencies you’ll need to include these in your build.gradle, too.