[network-controller] Add `lookupNetwork` timeout

[Gudahtt]

The NetworkController method lookupNetwork doesn't currently have a timeout. The default fetch timeout is quite long, leading to long delays in situations where we're waiting to know whether the network is accessible or not. We should add a timeout of 5 seconds. We can assume that networks that take longer than that to respond are inaccessible.

Acceptance Criteria

• Add a timeout on the lookupNetwork method
• Add unit tests for this change