title: android studio doesn't see sdk
body: <p>I today updated sdk as studio suggest,
  after that in such code</p>
  
  <pre><code>import android.app.Fragment;
  
      public class TaskFragment extends Fragment {
  </code></pre>
  
  <p>it show <code>Fragment</code> in red, and show hint <code>Cannot resolve symbol 'Fragment'</code>,
  but <code>Build-&gt;Make Project</code> without problem,
  how can I fix this issue?</p>
  
  <p><code>.AndroidStudio2.3/system/log/idea.log</code> rather big,
  so I post only error:</p>
  
  <pre><code>ERROR - aemon.impl.PassExecutorService - Accessing invalid virtual file: file:///home/xxx/Android/Sdk/sources/android-25; original:101; found:- 
  com.intellij.openapi.vfs.InvalidVirtualFileAccessException: Accessing invalid virtual file: file:///home/xxx/Android/Sdk/sources/android-25; original:101; found:-
  </code></pre>
  
  <p>directory <code>/home/xxx/Android/Sdk/sources/android-25</code> exists</p>