Fix google-explicit-constructor warnings in sensorservice.

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

Change-Id: Ie7c4dfcdbd069809e103f0dd008ae2fb4b0502f4
diff --git a/services/sensorservice/RingBuffer.h b/services/sensorservice/RingBuffer.h
index ec98a01..a60eb90 100644
--- a/services/sensorservice/RingBuffer.h
+++ b/services/sensorservice/RingBuffer.h
@@ -39,7 +39,7 @@
     /**
      * Construct a RingBuffer that can grow up to the given length.
      */
-    RingBuffer(size_t length);
+    explicit RingBuffer(size_t length);
 
     /**
      * Forward iterator to this class.  Implements an std:forward_iterator.