[SetupWizard] Refactor WifiSetupActivity

Refactor WifiSetupActivity to respect the conventional logic of Setup
Wizard's SubactivityWrapper. WifiSetupActivity now returns the result
code back to WifiSettingsWrapper and lets the Wrapper invoke
WizardManager, instead of invoking WizardManager by itself and letting
its Wrapper finish early.

The related change in WifiSettingsWrapper on Setup Wizard side is
ag/676788.

Bug: 20309008
Change-Id: Ia3bc2ff390404ff87c427782b5e7b345050bea03