title: MutableLiveData with BindingAdapter not updating visibility of view
body: <p>My BindingAdapter only runs once even though I update my LiveData multiple times.</p>
  
  <pre><code>public class ButtonViewBindingAdapter
  {
      @BindingAdapter("hideIfZero")
      public static void setHideIfZero(View view, MutableLiveData&lt;Integer&gt; currentPosition)
      {
          view.setVisibility(currentPosition.getValue() == 0 ? View.GONE : View.VISIBLE);
      }
  }
  </code></pre>
  
  <pre><code>&lt;layout xmlns:android="http://schemas.android.com/apk/res/android"
          xmlns:app="http://schemas.android.com/apk/res-auto"&gt;
  
      &lt;data&gt;
          &lt;variable
              name="viewModel"
              type="com.package.PermissionsViewModel"/&gt;
  
          &lt;variable
              name="clickHandler"
              type="com.package.PermissionsActivity.ClickHandler"/&gt;
      &lt;/data&gt;
  
      &lt;RelativeLayout
          android:layout_width="match_parent"
          android:layout_height="match_parent"&gt;
  
          &lt;Button
              android:id="@+id/buttonNext"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_alignParentRight="true"
              android:layout_alignParentBottom="true"
              android:background="@null"
              android:onClick="@{() -&gt; clickHandler.nextSlide()}"
              android:text="@string/intro_next"
              android:textColor="@android:color/white"
              /&gt;
  
          &lt;Button
              android:id="@+id/buttonBack"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_alignParentLeft="true"
              android:layout_alignParentBottom="true"
              android:background="@null"
              android:onClick="@{() -&gt; clickHandler.previousSlide()}"
              android:text="@string/intro_back"
              android:textColor="@android:color/white"
              app:hideIfZero="@{viewModel.currentSlidePosition}"/&gt;
  
      &lt;/RelativeLayout&gt;
  &lt;/layout&gt;
  </code></pre>
  
  <pre><code>
  public class PermissionsViewModel extends ViewModel
  {
      private MutableLiveData&lt;Integer&gt; currentSlidePosition = new MutableLiveData&lt;&gt;();
  
      public PermissionsViewModel()
      {
          currentSlidePosition.setValue(0);
      }
  
      public void setCurrentSlidePosition(final int position)
      {
          currentSlidePosition.setValue(position);
      }
  
      public MutableLiveData&lt;Integer&gt; getCurrentSlidePosition()
      {
          return currentSlidePosition;
      }
  }
  
  </code></pre>
  
  <p>When I update the value in my viewmodel the visibility does still not change. The BindingAdapter does not run more than once. What am I missing here? I want it to hide when the position is 0 otherwise it should show.</p>