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/NuPlayerDriver.h b/media/libmediaplayerservice/nuplayer/NuPlayerDriver.h
index 26d3a60..58008f0 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayerDriver.h
+++ b/media/libmediaplayerservice/nuplayer/NuPlayerDriver.h
@@ -24,7 +24,7 @@
 struct NuPlayer;
 
 struct NuPlayerDriver : public MediaPlayerInterface {
-    NuPlayerDriver(pid_t pid);
+    explicit NuPlayerDriver(pid_t pid);
 
     virtual status_t initCheck();