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/NuPlayerDecoderBase.h b/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
index a334ec5..9966144 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
+++ b/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
@@ -30,7 +30,7 @@
 class Surface;
 
 struct NuPlayer::DecoderBase : public AHandler {
-    DecoderBase(const sp<AMessage> &notify);
+    explicit DecoderBase(const sp<AMessage> &notify);
 
     void configure(const sp<AMessage> &format);
     void init();