AAPT2: Fix issue with deserializing binary XML

We assumed that a raw text value set for an attribute meant there
were no compiled values set either.

This would only really happen for attributes that did not belong to any
namespace (no prefix:), since we always kept their raw string values
in case some code relied on it.

Bug: 72700446
Test: make aapt2_tests
Change-Id: Icba40a1d4b181bfe7cad73131c4dbe5ba7f8b085