title: AAPT error after updating gradle
body: <p>Updated Gradle on an old project to latest version and I get this error: .gradle\caches\transforms-1\files-1.1\CreditCardEntry-1.4.8.10.aar\453339e1f1fb2382e8ce2362a0acdedd\res\values\values.xml:11:5-54: 
  AAPT: error:  inner element must either be a resource reference or empty.</p>
  
  <p>I can't figure out why this error appears and where to fix. </p>
  
  <p>Values.xml file in question: </p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;resources&gt;
  &lt;item name="null_color" type="color"&gt;234234&lt;/item&gt;
  &lt;color name="text_helper_color"&gt;#a8a8a8&lt;/color&gt;
  &lt;declare-styleable name="CreditCardForm"&gt;&lt;attr format="boolean" name="include_exp"/&gt;&lt;attr format="boolean" name="include_security"/&gt;&lt;attr format="boolean" name="include_zip"/&gt;&lt;attr format="boolean" name="include_helper"/&gt;&lt;attr format="color" name="helper_text_color"/&gt;&lt;attr format="color" name="text_color"/&gt;&lt;attr format="color" name="hint_text_color"/&gt;&lt;attr format="color" name="cursor_color"/&gt;&lt;attr format="string" name="card_number_hint"/&gt;&lt;attr format="reference" name="input_background"/&gt;&lt;attr format="boolean" name="default_text_colors"/&gt;&lt;attr format="boolean" name="animate_on_error"/&gt;&lt;attr format="dimension" name="text_size"/&gt;&lt;/declare-styleable&gt;
  &lt;item name="cc_card" type="id"&gt;2222&lt;/item&gt;
  &lt;item name="cc_ccv" type="id"&gt;4444&lt;/item&gt;
  &lt;item name="cc_entry" type="id"&gt;1000&lt;/item&gt;
  &lt;item name="cc_entry_internal" type="id"&gt;3000&lt;/item&gt;
  &lt;item name="cc_exp" type="id"&gt;3333&lt;/item&gt;
  &lt;item name="cc_form_layout" type="id"&gt;1111&lt;/item&gt;
  &lt;item name="cc_four_digits" type="id"&gt;6666&lt;/item&gt;
  &lt;item name="cc_zip" type="id"&gt;5555&lt;/item&gt;
  &lt;item name="text_helper" type="id"&gt;2000&lt;/item&gt;
  &lt;string name="CreditCardFormCardNumberHint"&gt;1234 5678 9012 3456&lt;/string&gt;
  &lt;string name="CreditCardNumberHelp"/&gt;
  &lt;string name="ExpDateFieldHint"&gt;MM/YY&lt;/string&gt;
  &lt;string name="ExpirationDateHelp"&gt;Expiration date (MM/YY)&lt;/string&gt;
  &lt;string name="SecurityCodeFieldHint"&gt;CVV&lt;/string&gt;
  &lt;string name="SecurityCodeHelp"&gt;Security code (CVV)&lt;/string&gt;
  &lt;string name="ZipCodeFieldHint"&gt;   ZIP   &lt;/string&gt;
  &lt;string name="ZipHelp"&gt;Zip code of billing address&lt;/string&gt;
  &lt;style name="AppBaseTheme" parent="android:Theme.Light"&gt;
      &lt;!--
          Theme customizations available in newer API levels can go in
          res/values-vXX/styles.xml, while customizations related to
          backward-compatibility can go here.
      --&gt;
  &lt;/style&gt;
  &lt;style name="AppTheme" parent="AppBaseTheme"&gt;
      &lt;!-- All customizations that are NOT specific to a particular API-level can go here. --&gt;
  &lt;/style&gt;
  </code></pre>
  
  <p></p>