Try this…
- Create libs folder under the application folder.
- Add .jar files to libs folder.
- Then add .jar files to app’s build.gradle dependency.
- Finally Sync project with Gradle files.
1.Create libs folder:

2.Add .jar to libs folder:

3.Edit app’s build.gradle dependency:
- Open app/build.gradle

4.Sync project with Gradle files:
- Finally add .jar files to your application.

UPDATE:
Here I’m going to import org.eclipse.paho.client.mqttv3.jar file to our app module.
- Copy your jar file and paste it in directory called libs.

- Press Ctrl + Alt + Shift + s or just click project structure icon on the toolbar.

- Then select your module to import .jar file, then select dependencies tab.

- Click plus icon then select File dependency

- Select .jar file path, click OK to build gradle.

- Finally we’re imported .jar file to our module.
