Add install-completed.conf

Part of a three CLs change that moves install-completed.conf from platform/init
to platform/installer.
Also moves crx-import.conf into the init directory to group the init scripts.

BUG=chromium:353400
TEST=Build chromeos-init and chromeos-installer. install-completed and
crx-import are both owned by chromeos-installer.

CQ-DEPEND=Ie874f191fc2ececd244469e4e6423a0d1c64df71
CQ-DEPEND=I5fd371c2bd9a20cd95a79b02525f88c179a1efd0

Change-Id: I0db3c3a932e654c95d77bbd1822b4fd011c0879f
Reviewed-on: https://chromium-review.googlesource.com/190433
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>