PackageParser: String interning fixes.

Stop interning string metadata values as well as class names as it's
unlikely there will be much duplication among these.

Also make sure we intern the same set of strings when parsing packages
from their cache entries as we do when parsing them from the package
itself.

This change also improves error reporting for the unit-test and fixes
a failure that was introduced by a previous change (the addition of
static libraries).

Test: PackageParserTest
Bug: 34726698

Change-Id: Ia0d0342b91b3294bd5569756255918d1dc886e05