How to define common android properties for all modules using gradle

You could create a build.gradle at the root of your project (i.e. the folder that contains all your modules), and use it to configure your rootProject.

For instance, if you have:

MyApp
  - Module1/
      - build.gradle
  - Module2/
      - build.gradle
  - settings.gradle

You can add a build.gradle next to settings.gradle.

In the example above you actually have 3 Gradle projects: Module1, Module2 and the rootProject.

So inside this build.gradle, you could do:

// use the ext object to add any properties to the project
project.ext {
   compileSdkVersion = 18
}

Then in your modules, you can do:

android {
    // here we reference the root project with the "rootProject" object.
    compileSdkVersion rootProject.ext.compileSdkVersion
}