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