AAPT2: Push more configuration code into the parser

When parsing is complete, we now have a list of output artifacts that
have their referential integrity validated. This means that once the
configuration file is parsed, the only errors that can occur are related
to APK processing, and not the configuration itself.

This reduces the number of errors that could cause a partial output of
APK artifacts. It simplifies the public API and reduces the complexity of
the code to generate multiple APKs.

Test: Ran unit tests
Test: manually ran the optimize command to ensure it still works

Change-Id: I3f2d885b207a84c958f5348a4baa6718598184a4