[Provider Model] Show searching sub-title only when the Wi-Fi network is empty

- Screenshot:
  https://screenshot.googleplex.com/7Z5r5AnpAZb6GfQ

Bug: 187886636
Test: manual test
atest -c InternetConnectivityPanelTest

Change-Id: Ib4769746819a5863e4a4981d0c73dda838ae3ae2

tests/unit/src/com/android/settings/panel/InternetConnectivityPanelTest.java

@@ -360,7 +360,6 @@ public class InternetConnectivityPanelTest {
 
         assertThat(mPanel.isProgressBarVisible()).isTrue();
         verify(mPanelContentCallback).onProgressBarVisibleChanged();
-        verify(mPanelContentCallback).onHeaderChanged();
         verify(mMainThreadHandler).postDelayed(any() /* mHideProgressBarRunnable */, anyLong());
     }