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.cpp b/services/surfaceflinger/Scheduler/Scheduler.cpp
index 4457f72..5b8cc10 100644
--- a/services/surfaceflinger/Scheduler/Scheduler.cpp
+++ b/services/surfaceflinger/Scheduler/Scheduler.cpp
@@ -133,7 +133,7 @@
     mConnections[handle->id]->thread->onScreenReleased();
 }
 
-void Scheduler::dump(const sp<Scheduler::ConnectionHandle>& handle, String8& result) const {
+void Scheduler::dump(const sp<Scheduler::ConnectionHandle>& handle, std::string& result) const {
     RETURN_IF_INVALID();
     mConnections.at(handle->id)->thread->dump(result);
 }