Make SF Hint Session safety margin adjustable with a debug prop

Bug: 271016792
Test: manual
Change-Id: I59615b90dc109285cd681b89ff80a569037c2992
(cherry picked from commit eae1e257b6dc5a039d5c8422b089849730e04c83)
diff --git a/services/surfaceflinger/DisplayHardware/PowerAdvisor.h b/services/surfaceflinger/DisplayHardware/PowerAdvisor.h
index d45e7cb..c4cfdc3 100644
--- a/services/surfaceflinger/DisplayHardware/PowerAdvisor.h
+++ b/services/surfaceflinger/DisplayHardware/PowerAdvisor.h
@@ -274,7 +274,8 @@
 
     // An adjustable safety margin which pads the "actual" value sent to PowerHAL,
     // encouraging more aggressive boosting to give SurfaceFlinger a larger margin for error
-    static constexpr const Duration kTargetSafetyMargin{1ms};
+    static const Duration sTargetSafetyMargin;
+    static constexpr const Duration kDefaultTargetSafetyMargin{1ms};
 
     // How long we expect hwc to run after the present call until it waits for the fence
     static constexpr const Duration kFenceWaitStartDelayValidated{150us};