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/tests/unittests/FlagManagerTest.cpp b/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp
index aa37754..c040f29 100644
--- a/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp
+++ b/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp
@@ -17,7 +17,7 @@
 #undef LOG_TAG
 #define LOG_TAG "FlagManagerTest"
 
-#include "FlagManager.h"
+#include <common/FlagManager.h>
 #include "FlagUtils.h"
 
 #include <gmock/gmock.h>