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
diff --git a/services/surfaceflinger/common/FlagManager.cpp b/services/surfaceflinger/common/FlagManager.cpp
index bd151be..f257c7c 100644
--- a/services/surfaceflinger/common/FlagManager.cpp
+++ b/services/surfaceflinger/common/FlagManager.cpp
@@ -115,6 +115,7 @@
     DUMP_ACONFIG_FLAG(adpf_gpu_sf);
     DUMP_ACONFIG_FLAG(adpf_native_session_manager);
     DUMP_ACONFIG_FLAG(adpf_use_fmq_channel);
+    DUMP_ACONFIG_FLAG(graphite_renderengine_preview_rollout);
 
     /// Trunk stable readonly flags ///
     DUMP_ACONFIG_FLAG(adpf_fmq_sf);
@@ -265,6 +266,7 @@
 FLAG_MANAGER_ACONFIG_FLAG(refresh_rate_overlay_on_external_display, "")
 FLAG_MANAGER_ACONFIG_FLAG(adpf_gpu_sf, "")
 FLAG_MANAGER_ACONFIG_FLAG(adpf_native_session_manager, "");
+FLAG_MANAGER_ACONFIG_FLAG(graphite_renderengine_preview_rollout, "");
 
 /// Trunk stable server (R/W) flags from outside SurfaceFlinger ///
 FLAG_MANAGER_ACONFIG_FLAG_IMPORTED(adpf_use_fmq_channel, "", android::os)