[SurfaceFlinger] add setDisplayContentSamplingEnabled i/f

Add interface to ISurfaceComposer that can enable or disable
the graphics.composer's collection of the displayed content
statistics.

Bug: 116028618
Test: Boot
Test: ran test client against prototype, see enable/disable working.
Test: Ran new tests './libgui_test --gtest_filter="DisplayedContentSamp*"'
Test: on hwc with and without new function hook.
Change-Id: Ifb487e2bfbd8e0db6178ccbf762aa968c34576b9
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 0375f74..3f1328e 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -131,6 +131,8 @@
     status_t getDisplayedContentSamplingAttributes(DisplayId displayId, ui::PixelFormat* outFormat,
                                                    ui::Dataspace* outDataspace,
                                                    uint8_t* outComponentMask);
+    status_t setDisplayContentSamplingEnabled(DisplayId displayId, bool enabled,
+                                              uint8_t componentMask, uint64_t maxFrames);
 
     // Events handling ---------------------------------------------------------