Enforce wifi config lockdown on details page.

Fixes the bug that, on the new WifiDetailPreference page, the "Forget"
button is not disabled properly for wifi configs created and protected
by DevicePolicyManger.

Robolectric test to follow in a separate CL.

Test: manual, by locking down wifi config with TestDPC
Test: make RunSettingsRoboTests
Bug: 64971700
Bug: 65396674
Change-Id: I27740eabd5eb94415e4258c9c80f91df2d9ab476