dhcp: decrease the lease time

According to the discussion, decrease the lease time for non-dev image
channels.

BUG=b/321300259
TEST=manual

Change-Id: I3bc00a6e6d460da1bfdba9172fa33568e43ca712
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/moblab/+/5376499
Reviewed-by: Michal Matyjek <mmatyjek@google.com>
Commit-Queue: Tomasz Jakubowski <jakubowskit@google.com>
Tested-by: Tomasz Jakubowski <jakubowskit@google.com>