Improvements to the key storage module

The key storage module didn't comply with Android coding standards
and had room for improvemnet in a few other ways, so have cleaned up.

Change-Id: I260ccff316423169cf887e538113b5ea400892f2
diff --git a/KeyStorage.h b/KeyStorage.h
index f3b7261..a35349c 100644
--- a/KeyStorage.h
+++ b/KeyStorage.h
@@ -26,13 +26,13 @@
 // in such a way that it can only be retrieved via Keymaster and
 // can be securely deleted.
 // It's safe to move/rename the directory after creation.
-bool StoreKey(const std::string &target_dir, const std::string &key);
+bool storeKey(const std::string &dir, const std::string &key);
 
 // Retrieve the key from the named directory.
-bool RetrieveKey(const std::string &dir, std::string &key);
+bool retrieveKey(const std::string &dir, std::string &key);
 
 // Securely destroy the key stored in the named directory and delete the directory.
-bool DestroyKey(const std::string &dir);
+bool destroyKey(const std::string &dir);
 
 }  // namespace vold
 }  // namespace android