Fix clang-tidy warnings in audio and playerservice.

* Add explicit keyword to conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
Test: build with WITH_TIDY=1

Change-Id: I265f3b094e08d5705b506b3fbba51439c134af84
diff --git a/include/media/RingBuffer.h b/include/media/RingBuffer.h
index df7c00e..4d92d87 100644
--- a/include/media/RingBuffer.h
+++ b/include/media/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.