Fix google-explicit-constructor warnings in  opengl.

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

Change-Id: I2508bb5c81e8fc7878a8ab5ee95cdf2c87729d85
diff --git a/opengl/tests/hwc/hwcTestLib.h b/opengl/tests/hwc/hwcTestLib.h
index a942c10..922fc19 100644
--- a/opengl/tests/hwc/hwcTestLib.h
+++ b/opengl/tests/hwc/hwcTestLib.h
@@ -71,7 +71,7 @@
 class ColorRGB {
   public:
     ColorRGB(): _r(0.0), _g(0.0), _b(0.0) {};
-    ColorRGB(float f): _r(f), _g(f), _b(f) {}; // Gray
+    ColorRGB(float f): _r(f), _g(f), _b(f) {}; // Gray, NOLINT(implicit)
     ColorRGB(float r, float g, float b): _r(r), _g(g), _b(b) {};
     float r(void) const { return _r; }
     float g(void) const { return _g; }