agl-compositor: RDEPENDS fixes

agl-compositor: RDEPENDS fixes

Changes:
- Add the required xkeyboard-config RDEPENDS to the agl-compositor
  recipe to avoid surprises if nothing else that pulls that in is
  in an image.
- Move the agl-compositor RDEPENDS on polkit directly into the
  recipe instead of getting indirectly via a meta-oe presence
  driven addition to packagegroup-agl-graphical-weston.  This is
  more correct, avoids surprises if that packagegroup happens to
  not be used, and will work better with a binary packagefeed.
  The RDEPENDS is still conditional on the presence of "polkit" in
  DISTRO_FEATURES, which seems a reasonable alternative to using
  dynamic-layers.

Bug-AGL: SPEC-4619

Change-Id: I595f55a9d598b4a15fe345f50d1a7a203616dd15
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28184
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>