Adds LoaderSettings-less constructors to UrlNetworkManager.

Main addition is the extra constructor which lists explicitly all the things required, without the help of LoaderSettings.

The LoaderSettings constructors are still available, and the relevant variables extracted from that, but deprecated.

A `newInstance()` method to return a `UrlNetworkManager` with default settings.

As headers are optional, a setter is provided, and is initialised to `Collections.emptyMap()` in the explicit constructor so it's never null.