title: The attribute is not declared (Android) on basic xml attributes
body: <p>I'm new to this Android development, and I find the layout of it really confusing. I'm trying to have a background image on a view, and I've tried using this example
  <a href="https://stackoverflow.com/questions/21002224/add-a-background-image-to-shape-in-xml-android">Add a background image to shape in xml Android</a>, but it looks very bad (bitmaps you know)</p>
  
  <p>So I thought vectors could be fun. Only problem is that I cannot get even the official examples to work. I've tried setting this as a background</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;vector xmlns:android="http://schemas.android.com/apk/res/android"
      android:height="256dp"
      android:width="256dp"
      android:viewportWidth="32"
      android:viewportHeight="32"&gt;
  &lt;path android:fillColor="#8fff" android:pathData="M20.5,9.5
                    c-1.955,0,-3.83,1.268,-4.5,3
                    c-0.67,-1.732,-2.547,-3,-4.5,-3
                    C8.957,9.5,7,11.432,7,14
                    c0,3.53,3.793,6.257,9,11.5
                    c5.207,-5.242,9,-7.97,9,-11.5
                    C25,11.432,23.043,9.5,20.5,9.5z" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>It renders in the design view and all, but the attributes <code>viewportWidth</code>, <code>viewportHeight</code>, <code>fillColor</code> and <code>pathData</code> all show the same warning:</p>
  
  <pre><code>The 'http://schemas.android.com/apk/res/android:viewportWidth' is not declared
  </code></pre>
  
  <p>If I check the file, sure enough, it's not there. Does that mean I have to explicitly declare all those types? It seems a bit odd for vanilla examples. </p>
  
  <p><a href="https://i.stack.imgur.com/H0rFq.png" rel="noreferrer"><img src="https://i.stack.imgur.com/H0rFq.png" alt=""></a></p>
  
  <p>Note that if I remove the 'android' in front of the warnings, it will remove warnings but still give me the same deployment error</p>
  
  <pre><code>Android.Views.InflateException: Binary XML file line #1: Error inflating class &lt;unknown&gt;
  </code></pre>