Use pointers not references for out arguments

Google/Android C++ style requires that arguments passed in for writing
should be pointers, not references, so that it's visible in the caller
that they'll be written to.

Bug: 27566014
Change-Id: I5cd55906cc4b2f61c8b97b223786be0b3ce28862
diff --git a/KeyStorage.h b/KeyStorage.h
index 54c3b2c..6daf1d2 100644
--- a/KeyStorage.h
+++ b/KeyStorage.h
@@ -42,7 +42,7 @@
 bool storeKey(const std::string &dir, const KeyAuthentication &auth, const std::string &key);
 
 // Retrieve the key from the named directory.
-bool retrieveKey(const std::string &dir, const KeyAuthentication &auth, std::string &key);
+bool retrieveKey(const std::string &dir, const KeyAuthentication &auth, std::string *key);
 
 // Securely destroy the key stored in the named directory and delete the directory.
 bool destroyKey(const std::string &dir);