Restrict Wi-Fi toggle in Internet Settings

- Disable Wi-Fi toggle when user is not allowed to change Wi-Fi state.

- Show restriction message in Wi-Fi toggle summary.

- See the result screenshot in b/203168097#comment24

Bug: 203168097
Test: manual test
atest -c WifiSwitchPreferenceControllerTest

Change-Id: I3cfe2f4f0e855dde91a82babe3a03005c3985d59