Multi packages per APK

This change introduces the ability to have multiple packages per
APK. The feature is currently restricted to privileged apps and
updates to such apps.

In essence the manifest can have multiple child package declarations.
A child package can declare everything an Android package can except
some tags or attributes that are not applicable and instead inherited
from the parent when needed. For example, the target SDK of the parent
applies to all children.

A child package can be updated only through the parent package.
A package with multiple child packages is installed, uninstalled
atomically - no partial installs where some child packages are not
installed.

The remaining work is to ensure broadcasts are also sent for child
packages. This will come in a subsequent change.

Sample app:ag/848432

Design doc: https://docs.google.com/document/d/18nFWtJuZchLxrHf5SBbJW03-Ky9Rh_G0-OVB14b6u78

Change-Id: I6fd021d981bf5786290e0c53502724a14c97358c