Show "Forget" button for ephemeral networks.

- Move forget button preference handling logic to
WifiDetailPreferenceController
- Remove unnecessary updateInfo call in onResume since
NETWORK_STATE_CHANGED_ACTION is a sticky broadcast
- Populate network state fields and set up initial wifi state
when the Preference is displayed instead of waiting for onResume

Bug: 37751816
Test: m RunSettingsRoboTests && manually tested on device
Change-Id: I509013c72a0d26fc9a3feca564d92a0ab8414d1e