Fix google-explicit-constructor warnings in system/netd.

Declare explicit conversion constructors.

Bug: 28341362
Change-Id: Iff77bfbd86c032fb335dd62203a3b387f9576f3a
Test: build with clang-tidy
diff --git a/server/DnsProxyListener.h b/server/DnsProxyListener.h
index bd7251d..b67ad5c 100644
--- a/server/DnsProxyListener.h
+++ b/server/DnsProxyListener.h
@@ -33,7 +33,7 @@
     const NetworkController *mNetCtrl;
     class GetAddrInfoCmd : public NetdCommand {
     public:
-        GetAddrInfoCmd(const DnsProxyListener* dnsProxyListener);
+        explicit GetAddrInfoCmd(const DnsProxyListener* dnsProxyListener);
         virtual ~GetAddrInfoCmd() {}
         int runCommand(SocketClient *c, int argc, char** argv);
     private:
@@ -65,7 +65,7 @@
     /* ------ gethostbyname ------*/
     class GetHostByNameCmd : public NetdCommand {
     public:
-        GetHostByNameCmd(const DnsProxyListener* dnsProxyListener);
+        explicit GetHostByNameCmd(const DnsProxyListener* dnsProxyListener);
         virtual ~GetHostByNameCmd() {}
         int runCommand(SocketClient *c, int argc, char** argv);
     private:
@@ -94,7 +94,7 @@
     /* ------ gethostbyaddr ------*/
     class GetHostByAddrCmd : public NetdCommand {
     public:
-        GetHostByAddrCmd(const DnsProxyListener* dnsProxyListener);
+        explicit GetHostByAddrCmd(const DnsProxyListener* dnsProxyListener);
         virtual ~GetHostByAddrCmd() {}
         int runCommand(SocketClient *c, int argc, char** argv);
     private:
diff --git a/server/NetdCommand.h b/server/NetdCommand.h
index 4f731bb..8e3e54b 100644
--- a/server/NetdCommand.h
+++ b/server/NetdCommand.h
@@ -21,7 +21,7 @@
 
 class NetdCommand : public FrameworkCommand {
 public:
-    NetdCommand(const char *cmd);
+    explicit NetdCommand(const char *cmd);
     virtual ~NetdCommand() {}
 };