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

This flag will allow some devices to opt in to RE-Graphite as a preview
rollout progresses. Making it a R/W flag will allow for A/B comparison.

Test: manual validation of aconfig flag / sysprop combos
Bug: b/293371537
Flag: com.android.graphics.surfaceflinger.flags.graphite_renderengine_preview_rollout
Change-Id: I30a59986d1da2dc8355c2b3cf2da436c6cc64120