[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/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index eff5fca..1dd6e23 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -483,6 +483,9 @@
     virtual status_t getDisplayedContentSamplingAttributes(
             const sp<IBinder>& display, ui::PixelFormat* outFormat, ui::Dataspace* outDataspace,
             uint8_t* outComponentMask) const override;
+    virtual status_t setDisplayContentSamplingEnabled(const sp<IBinder>& display, bool enable,
+                                                      uint8_t componentMask,
+                                                      uint64_t maxFrames) const override;
 
     /* ------------------------------------------------------------------------
      * DeathRecipient interface