Progress toward installer public API: callbacks.

Instead of surfacing all the existing cryptic error codes, we're
going to classify them into broad categories when surfacing through
public API.  This change introduces InstallResultCallback and
UninstallResultCallback, and wires them up to existing AIDL
interfaces.

Also start defining general SessionObserver for apps interested
in general progress details, such as Launcher apps.  Details about
active sessions are returned through new InstallSessionInfo objects.

Bug: 14975160
Change-Id: I068e2b0c30135f6340f59ae0fff93c321047f8f9