agl-cluster-demo-platform: Add databroker by default

Pull the KUKSA.val databroker into the agl-cluster-demo-platform
when building without the agl-demo-preload feature.  This matches
the behavior of the Flutter cluster demo image, and enables using
the image for standalone testing.

Bug-AGL: SPEC-4845

Change-Id: I8bc295858a28ea5cb842b01c5d58b59e3dc1e826
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29246
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>