Fix aapt2 JavaClassGenerator Maybe#value() call before check

Previous refactor accidentally assumed all attributes would have
resolvable symbols when generating the R class. Restore previous
behavior of calling SkipSymbol before retrieving the attribute
comment.

Bug: 145685954
Test: m TeleService
Change-Id: Ie8e1092556bc0b6a764462ffa0b99a876bcbd70e