AAPT2: Get list of multi-APK artifacts without APK file

Allow the optimize command to produce a list of output artifact names
without needing to read the input APK from disk. This not only saves
disk reads for a file that will not be used, it also allows the list
of outputs to be generated without having an APK file yet.

Test: unit tests
Test: split an APK
Test: print list of outputs without an APK
Change-Id: I4e18e63f298a07f26ab0de2b2f0acb769bb535ba