SECURITY: Always expand xml entities when checking SVG's

XmlTypeCheck's use of xml_parse for filtering SVG's sometimes left xml
entities unexpanded, which can lead to false-negatives when the
callback was used for filtering. Update XmlTypeCheck to use XMLReader
instead, tell the library to fully expand entities, and rely on the
library to error out if it encounters XML that is likely to cause a DoS
if parsed.

Bug: T88310
Change-Id: I77c77a2d6d22f549e7ef969811f7edd77a45dbba