[Provider Model] Refine sub-title (QS Internet Dialog)

- Show "No other networks available" when W+ connected only without
other WiFi networks

- See b/207470178#comment14 for a screenshot.

Bug: 207470178
Test: manual test
atest -c InternetDialogControllerTest

Change-Id: I7d2b5672d8e56b23aa81c1bc291abc2406b85287