Fix forget button on wifi details page.

- Corrected logic for forgetting the current network in the network
details page
- After a user has forgotten a network, the button becomes disabled
(TODO final UX on this - go/remaining-wifi-picker-work tracks this)
- Added unit tests for forgetting network flow
- Fixed bug where the current tethering WifiConfiguration was associated
with the currrent network

Bug: 36458941
Test: m RunSettingsRoboTests
Change-Id: Ie3c4bae8b295908b17179e60359c8bc599519744