[Provider Model] Show searching sub-title for 2 seconds

- When Wi-Fi entry is empty
  - Show searching sub-title for 2 seconds
  - Then show following sub-title
    - "No other networks available" if only Wi-Fi unavailable
    - "No networks available" if both Wi-Fi and mobile-data unavailable

Bug: 192818223
Bug: 192934101
Bug: 192935392
Test: manual test
atest InternetDialogTest \
      InternetDialogControllerTest

Change-Id: I8394f5808d776589cbf115e6df8a7d456df5bc17