NSApplication.sharedApplication is extension safe, so there is no need to eliminate the sharedApplication code from macOS embedder. For FlutterMacOS framework to work with extensions, we only need to add the fapplication-extension safe flag while building the framework.

When extension is running, the main bundle is different with running an app, so the FlutterDartProject.mm needs to be updated to search for FLTAssetsPath at the correct path.

Part of #124287