libhwui: RenderThread: fix UAF

Thread::run uses RefBase semantics and may delete the RenderThread out
from under the client.

Bug: 184196278
Test: no longer crashes when enhanced sp<> checks are enabled
Change-Id: I9dc306c14339b7142bae5f801970600d75221eb6
diff --git a/libs/hwui/renderthread/RenderThread.h b/libs/hwui/renderthread/RenderThread.h
index a7d1ba8..9e5bce7 100644
--- a/libs/hwui/renderthread/RenderThread.h
+++ b/libs/hwui/renderthread/RenderThread.h
@@ -171,6 +171,7 @@
     friend class android::uirenderer::WebViewFunctor;
     friend class android::uirenderer::skiapipeline::VkFunctorDrawHandler;
     friend class android::uirenderer::VectorDrawable::Tree;
+    friend class sp<RenderThread>;
 
     RenderThread();
     virtual ~RenderThread();