Slow progress towards APK clusters.

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