Fix google-explicit-constructor warnings in gui and ui.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I476ad1e9a4d47acb11bf4a87ad8baa855f441417
diff --git a/include/ui/Fence.h b/include/ui/Fence.h
index b431bd5..a4c1df7 100644
--- a/include/ui/Fence.h
+++ b/include/ui/Fence.h
@@ -53,7 +53,7 @@
     // Construct a new Fence object to manage a given fence file descriptor.
     // When the new Fence object is destructed the file descriptor will be
     // closed.
-    Fence(int fenceFd);
+    explicit Fence(int fenceFd);
 
     // Check whether the Fence has an open fence file descriptor. Most Fence
     // methods treat an invalid file descriptor just like a valid fence that