[Provider model] Revise the Internet Panel title sub-text

- Display "Tap a network to connect" on sub-title when Wi-Fi networks is
not empty

- Screenshot:
  https://screenshot.googleplex.com/5TzpJDLghpCQnDP

Bug: 188594439
Test: manual test
atest -c InternetConnectivityPanelTest

Change-Id: I0e3f0eb269fb4fe040a62be9ee84105879af82f7