[selected-network-controller] Track `metamask` selected domain

The `SelectedNetworkController` doesn't track the selected chain for the `metamask` domain, instead it relies on the NetworkController for this. As we make progress with eliminating reliance upon the "globally selected network", we want to move the `metamask` domain "current chain" tracking to the `SelectedNetworkController` so that all "selected chains" are tracked in the same place.

Additional details from @adonesky1 :

Migrate the (global) `selectedNetworkClientid` from the `NetworkController` to state in the domains object in the `SelectedNetworkController`.
- This will require updating selectors to point to this new location for this state.
- This will also require updating listeners.