[Provider Model] Add progress bar to internet panel

- Show progress bar when Wi-Fi scanning

- Show "Searching for networks..." sub-title when Wi-Fi scanning

- Remove "Wi-Fi" from searching networks string

Bug: 178774497
Test: manual test
atest -c InternetConnectivityPanelTest
make RunSettingsRoboTests ROBOTEST_FILTER=PanelFragmentTest

Change-Id: Ic05b939bef3b106845fe90db41eb09f0e15756f4

src/com/android/settings/panel/PanelContentCallback.java

@@ -42,4 +42,9 @@ public interface PanelContentCallback {
      * It will be called when panel requests to change the title.
      */
     void onTitleChanged();
+
+    /**
+     * It will be called when panel requests to change the progress bar visibility.
+     */
+    void onProgressBarVisibleChanged();
 }