Change the Endpoint for retrieving Platform Parameters

[Arjupta]

Currently the PlatformParameterService points to a demo endpoint for fetching platform parameters from the Oppia Backend. This url needs to be changed to a new endpoint after the platform parameter project gets completed. For doing so we need to first make the neccessary changes in the Oppia Backend to support the expected format of response needed and Afterwards we will raise a Pull request that changes the url in PlatformParameterService