AAPT2: Fix issue with String flattening in XmlFlattener

Compiled Strings (previously not encountered) in an XML resource
were using a different StringPool than the one being referred to
in the XmlFlattener, and so the indices were all wrong.

Bug: 72700446
Test: make aapt2_tests
Change-Id: I663924f8fad50fd4c69cfa196318dc63fb641a25