SurfaceFlinger: return DisplayStatInfo from getDisplayStatInfo

Code cleanup - return the class instead of passing it as an
output parameter.

Test: SF unit tests
Change-Id: I7a995415a81f3944d93a1cb573c8e3108464bd07
diff --git a/services/surfaceflinger/RegionSamplingThread.cpp b/services/surfaceflinger/RegionSamplingThread.cpp
index ad4877b..2aea8b4 100644
--- a/services/surfaceflinger/RegionSamplingThread.cpp
+++ b/services/surfaceflinger/RegionSamplingThread.cpp
@@ -250,8 +250,7 @@
         // If there is relatively little time left for surfaceflinger
         // until the next vsync deadline, defer this sampling work
         // to a later frame, when hopefully there will be more time.
-        DisplayStatInfo stats;
-        mScheduler.getDisplayStatInfo(&stats, systemTime());
+        const DisplayStatInfo stats = mScheduler.getDisplayStatInfo(systemTime());
         if (std::chrono::nanoseconds(stats.vsyncTime) - now < timeForRegionSampling) {
             ATRACE_INT(lumaSamplingStepTag, static_cast<int>(samplingStep::waitForQuietFrame));
             mDiscardedFrames++;