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/services/audiopolicy/enginedefault/src/Engine.h b/services/audiopolicy/enginedefault/src/Engine.h
index 606ad28..57538c4 100755
--- a/services/audiopolicy/enginedefault/src/Engine.h
+++ b/services/audiopolicy/enginedefault/src/Engine.h
@@ -43,7 +43,7 @@
     class ManagerInterfaceImpl : public AudioPolicyManagerInterface
     {
     public:
-        ManagerInterfaceImpl(Engine *policyEngine)
+        explicit ManagerInterfaceImpl(Engine *policyEngine)
             : mPolicyEngine(policyEngine) {}
 
         virtual void setObserver(AudioPolicyManagerObserver *observer)