AAPT2: Specify the output APK in log messages

Extend the context wrapper to allow it to also know the current artifact
that is being generated and to use that as a source for diagnostics
messages.

Test: ran unit tests
Test: manually ran optimize to split and APK and verified log messages

Change-Id: I1d66dacb38db58d57d2fffaa19f5fdaff9f908df