comparators.xml: fixes test_no_android_manifest from test_apk

This patch handles UnicodeDecodeError exception, which is caused
by decoding an invalid XML File. When this error is raised, XMLFile
returns False for recognizes() method, indicating for other object
calls that the given file is not a XMLFile.

Signed-off-by: Juliana Rodrigues <juliana.orod@gmail.com>