lorgnette: Add scanning

Add the ability to output scan data in responds to a DBus command.

BUG=chromium:239087
TEST=dbus-send --system --print-reply --fixed --dest=org.chromium.lorgnette \
  /org/chromium/lorgnette/Manager org.chromium.lorgnette.Manager.ScanImage \
  "string:genesys:libusb:001:012" "fd:1" "dict:string:variant:"

Change-Id: I74731e884c78aa4fc9beea99eee4fa99c5b92f1b
Reviewed-on: https://gerrit.chromium.org/gerrit/56311
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
3 files changed