Fix clang-tidy warnings in libs/hwui.

* Add explicit keyword to conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
Test: build with WITH_TIDY=1

Change-Id: Iab3e6636f60a70cb124f29dc19f20f842fa8dfda
diff --git a/libs/hwui/Image.h b/libs/hwui/Image.h
index 2514535..b8f5a5b 100644
--- a/libs/hwui/Image.h
+++ b/libs/hwui/Image.h
@@ -38,7 +38,7 @@
      * cannot be created, getTexture() will return 0 and getImage() will
      * return EGL_NO_IMAGE_KHR.
      */
-    Image(sp<GraphicBuffer> buffer);
+    explicit Image(sp<GraphicBuffer> buffer);
     ~Image();
 
     /**