[CrOS] Introduce feature flag for SoftwareUpdateScreen

Introduce a feature flag to enable SoftwareUpdateScreen
during OOBE.

Bug: b:283425050
Change-Id: Ib6fa73eccdf798a66d3d191a209420d941fb293d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548459


Reviewed-by: Osama Fathy <osamafathy@google.com>
Commit-Queue: Brahim Chikhaoui <bchikhaoui@google.com>
Cr-Commit-Position: refs/heads/main@{#1146481}