Update WriteWifiConfigToNfcDialog.

- Use the new WifiManager API to retrieve the WPS NFC token for the
current network.
- Add WifiManagerWrapper class to support testing because Robolectric
does not yet support testing Android O.

Bug: 35725168
Test: m RunSettingsRoboTests
Change-Id: I7805bdcbe02dc262083bcd371d4ad88d256f8089