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/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() {}
 };