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/media/libmediaplayerservice/nuplayer/NuPlayerCCDecoder.h b/media/libmediaplayerservice/nuplayer/NuPlayerCCDecoder.h
index a297334..f310f37 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayerCCDecoder.h
+++ b/media/libmediaplayerservice/nuplayer/NuPlayerCCDecoder.h
@@ -33,7 +33,7 @@
         kTrackTypeCEA708,
     };
 
-    CCDecoder(const sp<AMessage> &notify);
+    explicit CCDecoder(const sp<AMessage> &notify);
 
     size_t getTrackCount() const;
     sp<AMessage> getTrackInfo(size_t index) const;