Differentiate between "split APKs" and "cluster packages". A cluster
package is a directory containing zero or more APKs (base+splits),
and a monolithic package is a single APK (base).
PackageSetting will use the directory name as its codePath, so track
the baseCodePath separately. Clarify documentation in several
places.
Require that all installers provide file:// URIs through existing
hidden APIs; PackageInstaller hasn't been able to read content://
URIs for a long time.
Bug: 14975160
Change-Id: I1c6fed1b55205c2474b09871161a98a26669d22e
57dcf5b177