Add FrameEvents::isValidTimestamp

Refactor only.

Test: adb shell /data/nativetest/libgui_test/libgui_test
--gtest_filter=*GetFrameTimestamps*

Change-Id: I86653ac14186ca509f58a94047f25a7b74231b30
diff --git a/libs/gui/Surface.cpp b/libs/gui/Surface.cpp
index 45bf8c8..efb1524 100644
--- a/libs/gui/Surface.cpp
+++ b/libs/gui/Surface.cpp
@@ -230,7 +230,7 @@
 
 static void getFrameTimestamp(nsecs_t *dst, const nsecs_t& src) {
     if (dst != nullptr) {
-        *dst = Fence::isValidTimestamp(src) ? src : 0;
+        *dst = FrameEvents::isValidTimestamp(src) ? src : 0;
     }
 }