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/NuPlayer.cpp b/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
index 5846154..e370c2b 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
+++ b/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
@@ -428,7 +428,7 @@
 
 
 void NuPlayer::writeTrackInfo(
-        Parcel* reply, const sp<AMessage> format) const {
+        Parcel* reply, const sp<AMessage>& format) const {
     if (format == NULL) {
         ALOGE("NULL format");
         return;