Fix google-explicit-constructor warnings in libs/hwui.

* Add explicit keyword to conversion constructors,
  or add NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: Id8ca42433a4fc3652e4cb13787c4cb169f20d9a9
diff --git a/libs/hwui/FontRenderer.h b/libs/hwui/FontRenderer.h
index e10a81b..d656864 100644
--- a/libs/hwui/FontRenderer.h
+++ b/libs/hwui/FontRenderer.h
@@ -96,7 +96,7 @@
 
 class FontRenderer {
 public:
-    FontRenderer(const uint8_t* gammaTable);
+    explicit FontRenderer(const uint8_t* gammaTable);
     ~FontRenderer();
 
     void flushLargeCaches(std::vector<CacheTexture*>& cacheTextures);