title: angle attribute in android gradient
body: <p>I am going through test example. Where for some Image background they are using gradient,
  the code goes like this </p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  
  
    &lt;shape xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;gradient
          android:startColor="#ff0000"
          android:centerColor="#00ff00"
          android:endColor="#0000ff"
          android:angle="180"/&gt;
      &lt;corners android:radius="5dp" /&gt;
     &lt;/shape&gt;
  </code></pre>
  
  <p>In the above xml I didn't get <code>angle</code> attribute. but when I change the value of <code>angle</code> slightly the pattern slants. Can anyone explain me how exactly it works?</p>