Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I558551c6ab408e0b9bf278310857af4631769ec1
diff --git a/firewall_binder_client.cc b/firewall_binder_client.cc
index 1cc8e84..b29f49c 100644
--- a/firewall_binder_client.cc
+++ b/firewall_binder_client.cc
@@ -35,7 +35,7 @@
 
 class FirewallImpl : public Firewall {
  public:
-  FirewallImpl(sp<IBinder> server_firewall)
+  explicit FirewallImpl(sp<IBinder> server_firewall)
     : server_firewall_(android::interface_cast<IFirewall>(server_firewall)) {}
   virtual ~FirewallImpl() = default;