[SurfaceFlinger] add getDisplayedContentSample i/f

Add interface to ISurfaceComposer that can query the
graphics.composer for statistics on the displayed pixel content.

Bug: 116028618
Test: Boot
Test: ran test client, see data collected
Change-Id: Ide9b81b80c3399e7d648c7b611514e0d699120de
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index b27344d..dc6faad 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -770,6 +770,16 @@
     return NO_ERROR;
 }
 
+status_t HWComposer::getDisplayedContentSample(DisplayId displayId, uint64_t maxFrames,
+                                               uint64_t timestamp, DisplayedFrameStats* outStats) {
+    RETURN_IF_INVALID_DISPLAY(displayId, BAD_INDEX);
+    const auto error =
+            mDisplayData[displayId].hwcDisplay->getDisplayedContentSample(maxFrames, timestamp,
+                                                                          outStats);
+    RETURN_IF_HWC_ERROR(error, displayId, UNKNOWN_ERROR);
+    return NO_ERROR;
+}
+
 bool HWComposer::isUsingVrComposer() const {
     return getComposer()->isUsingVrComposer();
 }