Investigate removing systemd user session use

At this point we only use the user session in systemd for the compositor, homescreen/launcher on IVI, and cluster apps.  There are a few reasons why going ahead and removing it in favor of system units with explicit User configuration:

• User session units cannot use most of the systemd sandboxing functionality.
• System units cannot have user units as dependencies, only the overall user session, which makes it difficult to have robust dependencies in some configurations.
• At present the addition of the user session systemd configuration done by meta-app-framework results in simple images such as agl-image-weston no longer being usefully buildable (e.g. if building with agl-demo), which complicates test building images.

The intent of this issue is to cover the investigation and possible change to drop using the user session.