Add FlagManager instance to SF.

Bug: 190769260
Test: make;

1. Examine the value returned by FlagManager:
   `adb shell dumpsys SurfaceFlinger | grep -A 10 "FlagManager"`
2. Examine the value returned by P/H:
   `adb shell device_config get surface_flinger_native_boot
   DemoFeature__demo_flag`

Change-Id: Ied375fa732c33be31cc5e1c6dcdfb024aecb3e0d
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index e335e56..ba8aca3 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -89,6 +89,7 @@
 
 class Client;
 class EventThread;
+class FlagManager;
 class FpsReporter;
 class TunnelModeEnabledReporter;
 class HdrLayerInfoReporter;
@@ -1516,6 +1517,8 @@
     wp<IBinder> mActiveDisplayToken GUARDED_BY(mStateLock);
 
     const sp<WindowInfosListenerInvoker> mWindowInfosListenerInvoker;
+
+    std::unique_ptr<FlagManager> mFlagManager;
 };
 
 } // namespace android