[Expressive Design] Retain the initial 25Q2 release background color for v36 theme

Because it is not safe to ship a color update via Mainline to any
released platform, while the old color is in 25Q2 release right now.

To do such updates safely, they need to require an opt-in from the
platform, e.g. only be done when an exported feature flag hosted in the
platform is enabled.

Bug: 408996309
Test; manual
Flag: EXEMPT bug fix
Change-Id: Ie6119665c76176a5db9c765bd350910a575bf576