SurfaceFlinger: Get rid of String8 in dumpsys

Test: build and gui/ui/renderengine/surfaceflinger tests and compare
dumpsys SurfaceFlinger results
Change-Id: Iafdaf73a0acbbbb8fbafc930ad86285106a07e13
diff --git a/services/surfaceflinger/Scheduler/Scheduler.h b/services/surfaceflinger/Scheduler/Scheduler.h
index 764ad00..ea90824 100644
--- a/services/surfaceflinger/Scheduler/Scheduler.h
+++ b/services/surfaceflinger/Scheduler/Scheduler.h
@@ -91,7 +91,7 @@
     void onScreenReleased(const sp<ConnectionHandle>& handle);
 
     // Should be called when dumpsys command is received.
-    void dump(const sp<ConnectionHandle>& handle, String8& result) const;
+    void dump(const sp<ConnectionHandle>& handle, std::string& result) const;
 
     // Offers ability to modify phase offset in the event thread.
     void setPhaseOffset(const sp<ConnectionHandle>& handle, nsecs_t phaseOffset);