Add sysprop to control HDR headroom

To accelerate tuning of the value allow it to be sysprop driven

Bug: 273624684
Test: change prop, verify silkfx headroom changes after re-launch
Change-Id: Ibad662b7ba06bd821c7a3ce48c19f4c94c716b5e
diff --git a/libs/hwui/renderthread/CanvasContext.cpp b/libs/hwui/renderthread/CanvasContext.cpp
index f10b2b2..dd781bb 100644
--- a/libs/hwui/renderthread/CanvasContext.cpp
+++ b/libs/hwui/renderthread/CanvasContext.cpp
@@ -311,7 +311,7 @@
     }
     switch (mColorMode) {
         case ColorMode::Hdr:
-            return 3.f;  // TODO: Refine this number
+            return Properties::maxHdrHeadroomOn8bit;
         case ColorMode::Hdr10:
             return 10.f;
         default: