Demo mode: Detect extension IDs via switches

In CL:2902417, chromeos-config was updated to pass the demo mode
extension IDs (specified in model.yaml) via switches to the chrome
process:

  --demo-mode-highlights-extension
  --demo-mode-screensaver-extension

Update the demo session code to use these switches instead of the
previously hardcoded board-name/HWID detection logic.

Bug: b/188476794
Change-Id: Ifa8519b26e7ed27ee486fd48042fc76bc4da4d4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2904427
Auto-Submit: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Yves Arrouye <drcrash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#884501}