Restrict WifiDppConfiguratorActivity

- Don't show WifiDppConfiguratorActivity if user has DISALLOW_ADD_WIFI_CONFIG

- Don't show AddNetworkFragment if user has DISALLOW_ADD_WIFI_CONFIG

Fix: 299931076
Flag: None
Test: manual test with TestDPC
atest -c SettingsUnitTests:AddNetworkFragmentTest \
         SettingsUnitTests:WifiDppConfiguratorActivityTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2101258e8a21e7c983c6b000209f60c22bb3bfd4)
Merged-In: I34afe0f698e2dc43eba59b25f5f3f4f61e70166a
Change-Id: I34afe0f698e2dc43eba59b25f5f3f4f61e70166a