Fix google-explicit-constructor warnings in keystore.

Declare explicit conversion constructors.

Bug: 28341362
Change-Id: Ibd0d6b7ec63ccb8f910e9da15771ff4d0031f4c4
Test: build with clang-tidy
diff --git a/keystore/key_store_service.h b/keystore/key_store_service.h
index 35b8928..e744d1c 100644
--- a/keystore/key_store_service.h
+++ b/keystore/key_store_service.h
@@ -29,7 +29,7 @@
 
 class KeyStoreService : public BnKeystoreService, public IBinder::DeathRecipient {
   public:
-    KeyStoreService(KeyStore* keyStore) : mKeyStore(keyStore), mOperationMap(this) {}
+    explicit KeyStoreService(KeyStore* keyStore) : mKeyStore(keyStore), mOperationMap(this) {}
 
     void binderDied(const wp<IBinder>& who);