Fix google-explicit-constructor warnings in frameworks/base

* Add explicit keyword to conversion constructors.
* Add NOLINT to implicit conversion constructors.

Bug: 28341362
Test: build with clang-tidy
Change-Id: Ie4d37072ab57d1662d18db4de1c8577247f43337
diff --git a/libs/hwui/GradientCache.h b/libs/hwui/GradientCache.h
index 08319ea..54df9bf 100644
--- a/libs/hwui/GradientCache.h
+++ b/libs/hwui/GradientCache.h
@@ -104,7 +104,7 @@
  */
 class GradientCache: public OnEntryRemoved<GradientCacheEntry, Texture*> {
 public:
-    GradientCache(Extensions& extensions);
+    explicit GradientCache(Extensions& extensions);
     ~GradientCache();
 
     /**