Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I116d2667550eec7a080bd5fcf073a8e7206c7c53
diff --git a/keystore/IKeystoreService.cpp b/keystore/IKeystoreService.cpp
index 2753627..635d3a7 100644
--- a/keystore/IKeystoreService.cpp
+++ b/keystore/IKeystoreService.cpp
@@ -456,7 +456,7 @@
 class BpKeystoreService: public BpInterface<IKeystoreService>
 {
 public:
-    BpKeystoreService(const sp<IBinder>& impl)
+    explicit BpKeystoreService(const sp<IBinder>& impl)
         : BpInterface<IKeystoreService>(impl)
     {
     }