AAPT2: Treat non-AAPT attributes as Attributes with no format specified.

Attributes in XML like "class" should be treated as attributes with no
format specified. That means that something like 'version="1"' will
be interpreted as an Integer value instead of as a string.

This is more in line with the old AAPT behaviour.
Bug: 30763349
Test: make libaapt2_tests

Change-Id: I02fe41617e3c62c354714056198ef480d62afee7