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/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index 808b3ab..33974e0 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -1166,7 +1166,7 @@
     return aps->getPhoneState();
 }
 
-status_t AudioSystem::registerPolicyMixes(Vector<AudioMix> mixes, bool registration)
+status_t AudioSystem::registerPolicyMixes(const Vector<AudioMix>& mixes, bool registration)
 {
     const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
     if (aps == 0) return PERMISSION_DENIED;