Move FlagManager to a common SF lib

So that CompositionEngine can use it.

Bug: 236745178
Test: builds
Change-Id: Ib873cb4a0b4231723a22fbc8fa22ec955aa6f329
diff --git a/services/surfaceflinger/RefreshRateOverlay.cpp b/services/surfaceflinger/RefreshRateOverlay.cpp
index 6752a0b..b960e33 100644
--- a/services/surfaceflinger/RefreshRateOverlay.cpp
+++ b/services/surfaceflinger/RefreshRateOverlay.cpp
@@ -16,8 +16,8 @@
 
 #include <algorithm>
 
+#include <common/FlagManager.h>
 #include "Client.h"
-#include "FlagManager.h"
 #include "Layer.h"
 #include "RefreshRateOverlay.h"