Add useVrFlinger() method to SurfaceFlinger ConfigStore

Add useVrFlinger() method to SurfaceFlinger ConfigStore, which is
used to ensure we only start vr flinger on Daydream ready devices.

Bug: 35885165

Test: Checked for "SurfaceFlinger UseVrFlinger" in the logs,
confirmed it only shows up for Daydream ready devices.

Change-Id: I3c39b1f094e28f06b510171fc0968cf71f5d5550
diff --git a/configstore/1.0/default/surfaceflinger.mk b/configstore/1.0/default/surfaceflinger.mk
index 6e63fdc..7837498 100644
--- a/configstore/1.0/default/surfaceflinger.mk
+++ b/configstore/1.0/default/surfaceflinger.mk
@@ -46,3 +46,7 @@
 ifeq ($(TARGET_RUNNING_WITHOUT_SYNC_FRAMEWORK),true)
     LOCAL_CFLAGS += -DRUNNING_WITHOUT_SYNC_FRAMEWORK
 endif
+
+ifneq ($(USE_VR_FLINGER),)
+    LOCAL_CFLAGS += -DUSE_VR_FLINGER
+endif