Rename AndroidPackageApi to AndroidPackage

Makes it so that the non-suffixed name is the to-be system API,
renaming the internal class with an _Internal suffix.

This will also happen to all other exposed internal package data
types in future changes.

Test: presubmit, just a naming change

Change-Id: I8695e7f1644ff5af9303bb32ddec3480bd155bf8