title: PercentRelativeLayout - layout_width missing warning
body: <p>I'm trying out PercentRelativeLayout from the support library, and <a href="http://developer.android.com/reference/android/support/percent/PercentRelativeLayout.html">the docs</a> just ask me to specify an app:layout_widthPercent property. However, when I do that, Android Studio gives me a red warn saying that the layout_width is unspecified.</p>
  
  <p>Other than suppressing the warning, is there a way around this?</p>
  
  <p>For example:</p>
  
  <pre><code>    &lt;android.support.percent.PercentRelativeLayout
          android:layout_width="match_parent"
          android:layout_height="wrap_content"&gt;
  
          &lt;android.support.design.widget.TextInputLayout
              android:id="@+id/expiry_month_wrapper"
              app:layout_widthPercent="25%"
              android:layout_height="wrap_content"&gt;
              &lt;EditText
                  android:id="@+id/expiry_month_editText"
                  android:layout_width="match_parent"
                  android:layout_height="wrap_content"
                  android:maxLength="2"
                  android:hint="@string/month"
                  android:focusable="true"
                  android:inputType="number"/&gt;
          &lt;/android.support.design.widget.TextInputLayout&gt;
  
        &lt;!-- more TextInputLayout fields --&gt;
  
      &lt;/android.support.percent.PercentRelativeLayout&gt;
  </code></pre>
  
  <p>Warning given on TextInputLayout.</p>