Add automatic HDR transition animation

Test: enable flag && silkfx
Bug: 314810174

Change-Id: Ic69b633d6042145a537c7d39e713f804faff6600
diff --git a/libs/hwui/aconfig/hwui_flags.aconfig b/libs/hwui/aconfig/hwui_flags.aconfig
index 78a6479..a54a039 100644
--- a/libs/hwui/aconfig/hwui_flags.aconfig
+++ b/libs/hwui/aconfig/hwui_flags.aconfig
@@ -41,3 +41,10 @@
   description: "Enable r_8, r_16_uint, rg_1616_uint, and rgba_10101010 in the SDK"
   bug: "292545615"
 }
+
+flag {
+  name: "animate_hdr_transitions"
+  namespace: "core_graphics"
+  description: "Automatically animate all changes in HDR headroom"
+  bug: "314810174"
+}