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

rsElement.h

diff --git a/rsElement.h b/rsElement.h
index 9374c64..d8f101b 100644
--- a/rsElement.h
+++ b/rsElement.h
@@ -152,7 +152,7 @@
 
 
     virtual ~Element();
-    Element(Context *);
+    explicit Element(Context *);
 
     Component mComponent;
     uint32_t mBitsUnpadded;