[wpilibc] Return reference from GetInstance (#3247)

Improves consistency across all classes.

Affects Preferences, LiveWindow, and CameraServer.

Old commands Scheduler::GetInstance() was not updated as this is already
deprecated.

cameraserver/src/main/native/cpp/cameraserver/CameraServer.cpp

@@ -45,7 +45,7 @@ struct CameraServer::Impl {
   std::vector<std::string> m_addresses;
 };
 
-CameraServer* CameraServer::GetInstance() {
+CameraServer& CameraServer::GetInstance() {
   struct Creator {
     static void* call() { return new CameraServer{}; }
   };
@@ -53,7 +53,7 @@ CameraServer* CameraServer::GetInstance() {
     static void call(void* ptr) { delete static_cast<CameraServer*>(ptr); }
   };
   static wpi::ManagedStatic<CameraServer, Creator, Deleter> instance;
-  return &(*instance);
+  return *instance;
 }
 
 static wpi::StringRef MakeSourceValue(CS_Source source,

cameraserver/src/main/native/include/cameraserver/CameraServer.h

@@ -32,7 +32,7 @@ class CameraServer {
   /**
    * Get the CameraServer instance.
    */
-  static CameraServer* GetInstance();
+  static CameraServer& GetInstance();
 
   /**
    * Start automatically capturing images to send to the dashboard.