AAPT2: Fix processing of quotes in XML

When processing attributes in XML, quotes can't be used to mark a
section as whitespace preserving, so the assumption should be that the
entire string is whitespace preserving, which makes quote characters
literals.

Bug: 62840718
Bug: 62840406
Test: make aapt2_tests
Change-Id: I4afff02148b5b8e78833abf1f323c2f5325d6155