Add support for attributes defined in res-auto namespace

The "http://schemas.android.com/apk/res-auto" namespace in layout XML files is replaced with the manifest package name at build time by aapt.
Robolectric doesn't support this, causing attribute lookups in the res-auto namespace to fail.

This pull request adds some processing to the XML file loading. It goes through all the Nodes in the resource Document, looking for the res-auto namespace and replacing it.

Another way to solve the problem is by recognizing the res-auto namespace when looking up attributes, as in https://github.com/bastiaanve/robolectric/commit/ad7ff6092f72630bb9202c575b0626cf1ac54b3e

Please comment which approach is preferred.