Switch PackageParser to reference single path.

It previously kept mPath separate from mScanPath for some very odd
edge cases around moving apps-on-SD.  This changes it to always use
a single path, refactors moving to keep separate paths.

Refactors method names in PackageParser to be clearer about their
APK-versus-package relationship.

Beginnings of a split package parser.  Instead of requiring that
callers check error codes when null, switch to always throwing on
parse errors, to require that callers deal with the error.  Longer
term the entire parser should switch to this style, but its too
pervasive for a simple refactoring.

Change-Id: If071d8e55e46e56cc201fadfb51cb471713ae973