WifiManager: Mark all settings/setupwizard custom APIs @SystemApi

Mark all the custom connect (& other APIs) added to serve settings &
setupwizard as @SystemApi.

Also, enforce tighter permissions on these APIs. These were explicitly
added to serve settings & we should not allow any other apps to use
them.

Bug: 116127346
Test: Compiles

Change-Id: Ic0000f21ccbeda4ade59a3b04d78f49b1b20cd02