[selected-network-controller] Track metamask selected domain

[Gudahtt]

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.

[adonesky1]

This was by design at first since we weren't ready to migrate that state. But we should still do it now. I don't see any reason for it to remain blocked:

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.

[Gudahtt]

This was by design at first since we weren't ready to migrate that state

Ah, sorry I misunderstood how this worked. I thought it was set post-initialization, but it's currently never set, so this isn't really a bug. I'll update the description to reflect the work you mentioned.