[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/HWC2.cpp b/services/surfaceflinger/DisplayHardware/HWC2.cpp
index 733a5da..d2aa4ad 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2.cpp
@@ -562,6 +562,12 @@
     return static_cast<Error>(intError);
 }
 
+Error Display::getDisplayedContentSample(uint64_t maxFrames, uint64_t timestamp,
+                                         android::DisplayedFrameStats* outStats) const {
+    auto intError = mComposer.getDisplayedContentSample(mId, maxFrames, timestamp, outStats);
+    return static_cast<Error>(intError);
+}
+
 Error Display::getReleaseFences(
         std::unordered_map<Layer*, sp<Fence>>* outFences) const
 {