title: android build tool adds v4 qualifier to drawable folders by default in generated apk
body: <p>I have been confused for quite a long time.</p>
  
  <p>before build tool v21.0.0, the packaged apk contains the drawable folder structure as below:</p>
  
  <pre><code>res/drawable-hdpi
  res/drawable-ldpi
  res/drawable-mdpi
  res/drawable-hdpi
  </code></pre>
  
  <p>However, things changed since v21.0.0, the unzip folder structure comes with suffix:</p>
  
  <pre><code>res/drawable-hdpi-v4
  res/drawable-ldpi-v4
  res/drawable-mdpi-v4
  res/drawable-hdpi-v4
  </code></pre>
  
  <p>My questions are:</p>
  
  <ol>
  <li>Why added v4 suffix to the drawable folders by default? In current days, the apps are normally build on a much higher version of API. I don't feel like it is necessary to add the v4 qualifier.</li>
  <li>Why since v21.0.0? I have gone through almost all the version of build tools and this only happens since v21.0.0.</li>
  <li>Will there be another qualifiers like v6, v9 or etc in the future?</li>
  </ol>
  
  <p>Thank you so much in advance.</p>