First version of Staging Manager.

This CL introduces a new class called Staging Manager, which PackageInstaller
uses to delegate the management of staged session, i.e. to coordinate with
apexd and PackageManager to perform package verification and
installation at reboot.

In this first version I ported a mock implementation of the staging
session management.

Bug: 118865310
Fix: 122072686
Test: Used a small app to query getStagedSessions and to receive the
broadcast messaged. Installed package with adb install --staged.
Change-Id: Iaeda1e8bee8d375b78bb0156baa8e7e53e5e23c1