Tie VR mode support to a property

Bug: 35854545
Test:
  - Validate VR mode is not supported when ro.boot.vr is unset
  - Validate VR mode is supported when ro.boot.vr is set

Change-Id: I26f7e851766eab0d8a5a2f9fb4d72bfcee0c42f3
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 0b3deef..d63c0bb 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -557,6 +557,7 @@
     DefaultKeyedVector< wp<IBinder>, sp<DisplayDevice> > mDisplays;
 
     // don't use a lock for these, we don't care
+    int mVrModeSupported;
     int mDebugRegion;
     int mDebugDDMS;
     int mDebugDisableHWC;