Restrict WifiDialogActivity

- Don't show WifiDialogActivity if user has DISALLOW_ADD_WIFI_CONFIG

Fix: 299931761
Bug: 314867581
Flag: None
Test: manual test with TestDPC
atest -c SettingsRoboTests:WifiDialogActivityTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:51fa3d798ad0397122bbb2143bc24efe1a705be9)
Merged-In: Icbb8f45922ded163208976be9c2816060dcf09f1
Change-Id: Icbb8f45922ded163208976be9c2816060dcf09f1