Add r/w opt-in rollout flag for RenderEngine Graphite on Desktop

Essentially the same flag as the existing graphite_renderengine_preview_rollout
but this flag targets RE-Graphite on Desktop devices that have a
different rollout timeline.

Test: aconfig and sysprop combos
Bug: 436872839
Flag: com.android.graphics.surfaceflinger.flags.graphite_renderengine_desktop_rollout
Change-Id: Ifc36f47f13adad46ba9bc05dc4bb52674f207a13