SF: Remove manual enum stringification

Upgrade to scoped enums where applicable. Pull GameMode from TimeStats
into libgui to plumb it as an enum rather than int32_t.

Bug: 185536303
Test: libsurfaceflinger_unittest
Change-Id: I81fdd24805757ef953484055ee867684eb94fecf
diff --git a/services/surfaceflinger/Scheduler/Timer.h b/services/surfaceflinger/Scheduler/Timer.h
index 69ce079..628d800 100644
--- a/services/surfaceflinger/Scheduler/Timer.h
+++ b/services/surfaceflinger/Scheduler/Timer.h
@@ -37,11 +37,20 @@
     void dump(std::string& result) const final;
 
 private:
-    enum class DebugState { Reset, Running, Waiting, Reading, InCallback, Terminated };
+    enum class DebugState {
+        Reset,
+        Running,
+        Waiting,
+        Reading,
+        InCallback,
+        Terminated,
+
+        ftl_last = Terminated
+    };
+
     void reset();
     void cleanup();
     void setDebugState(DebugState state) EXCLUDES(mMutex);
-    const char* strDebugState(DebugState state) const;
 
     int mTimerFd = -1;
     int mEpollFd = -1;