crypto: Make KeyManager a singleton class

Previously, we were reading the keys everytime a KeyManager object was created, causing yuzu to reread the keys file multiple hundreds of times when loading the game list.
With this change, it is only loaded once.
On my system, this decreased game list loading times by a factor of 20.

src/core/hle/service/es/es.cpp

@@ -263,7 +263,7 @@ private:
         rb.Push<u64>(write_size);
     }
 
-    Core::Crypto::KeyManager keys;
+    Core::Crypto::KeyManager& keys = Core::Crypto::KeyManager::instance();
 };
 
 void InstallInterfaces(SM::ServiceManager& service_manager) {