title: Create sample flutter plugin to import AndroidX and get error cannot resolve symbol 'XXX'
body: <p>I have created a sample flutter plugin and follow the AndroidX compatibility document to upgrade plugin to AndroidX <br>
  1. Edit Android Code in Android Studio <br>
  2. Use IDE Migrating to AndroidX <br>
  3. classpath is still 'com.android.tools.build:gradle:3.2.0', manually change to 3.3.0 <br>
  4. gradle.properties has</p>
  
  <pre><code>android.enableJetifier=true
  android.useAndroidX=true 
  </code></pre>
  
  <p>appended by IDE <br>
  5. </p>
  
  <pre><code>testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner" 
  </code></pre>
  
  <p>has changed by IDE <br>
  6. </p>
  
  <pre><code>androidTestImplementation 'androidx.test:runner:1.1.1'
  androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1' 
  </code></pre>
  
  <p>has changed by IDE <br>
  7. after that I add a Java class "ImagePickerFileProvider" copied from Image Picker Plugin, but still get error "Cannot resolve symbol 'FileProvider'"</p>
  
  <pre><code>package xxx.flutter_plugin_androidx_test;
  
  import androidx.core.content.FileProvider;
  
  /**
   * Providing a custom {@code FileProvider} prevents manifest {@code &lt;provider&gt;} name collisions.
   *
   * &lt;p&gt;See https://developer.android.com/guide/topics/manifest/provider-element.html for details.
   */
  public class ImagePickerFileProvider extends FileProvider {}
  </code></pre>