Fix google-explicit-constructor warnings in frameworks/rs.

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

 Bug: 28341362
 Test: build with clang-tidy

Change-Id: If5114de668c1c8946b8fca9cee4b2b14f95be5f1
diff --git a/rsSampler.h b/rsSampler.h
index 2fdf707..c63a4a8 100644
--- a/rsSampler.h
+++ b/rsSampler.h
@@ -74,7 +74,7 @@
     virtual ~Sampler();
 
 private:
-    Sampler(Context *);
+    explicit Sampler(Context *);
     Sampler(Context *,
             RsSamplerValue magFilter,
             RsSamplerValue minFilter,