Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/binder/IServiceManager.cpp b/libs/binder/IServiceManager.cpp
index 725ebdf..2062b3b 100644
--- a/libs/binder/IServiceManager.cpp
+++ b/libs/binder/IServiceManager.cpp
@@ -126,7 +126,7 @@
 class BpServiceManager : public BpInterface<IServiceManager>
 {
 public:
-    BpServiceManager(const sp<IBinder>& impl)
+    explicit BpServiceManager(const sp<IBinder>& impl)
         : BpInterface<IServiceManager>(impl)
     {
     }