Run agl-compositor in system scope for QEMU+KVM demo image

Run agl-compositor in system scope for QEMU+KVM demo image

To allow usefully depending on the native-shell-client "homescreen"
application from the systemd units running QEMU, tweak things to
drop the use of agl-session from the agl-kvm-demo-platform host
image.  The motivation for this is to have the VM guest start up
ordering work as expected, avoiding unnecessary delays from race
failures on boot.

Changes:
- Rework agl-kvm template to use conf/include/agl-kvm.inc include
  file to match other featurs and simplify development.
- Add include files for the host ("default" in BitBake) and guest
  (so agl-kvm-guest) multiconfigs to allow disabling the agl-demo
  feature in the host build, which disables agl-session.
- Update native-shell-client systemd unit to run at system scope
  and depend upon agl-compositor directly.
- Update agl-qemu-runner systemd template unit to depend directly
  on native-shell-client.

Note that a consequence of these changes is that building images
other than agl-kvm-demo-platform when the "agl-kvm" feature is used
will likely not yield the desired results.  Future work will
investigate avoiding this problem, or forcing explicit compatibility
errors to avoid user confusion.

Bug-AGL: SPEC-4618

Change-Id: I7f2f2ad9a453b5d99bed4cfc33d99eb3002bcceb
Signed-off-by: Scott Murray <scott.murray@konsulko.com>