title: Android Studio don't generate R.java for my import project
body: <p>I import a project to Android Studio , but the R.java is always empty.</p>
  
  <pre><code>public final class R {
  }
  </code></pre>
  
  <p>I have tried:</p>
  
  <ol>
  <li>Make project,make module, remake, etc.</li>
  <li>Project Setting-modules-sources, then add src and gen as my source folder.</li>
  </ol>
  
  <p>But they are not take effect.</p>