[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/PanelContent.java

@@ -110,4 +110,11 @@ public interface PanelContent extends Instrumentable {
     default int getViewType() {
         return 0;
     }
+
+    /**
+     * @return {@code true} to enable progress bar visibility, {@code false} otherwise.
+     */
+    default boolean isProgressBarVisible() {
+        return false;
+    }
 }