Bring back "Turning on hotspot" status text when starting hotspot

We accidentally regressed showing this text when starting the hotspot in
the refactor ag/2381595.

Bug: 64811203
Test: make RunSettingsRoboTests

src/com/android/settings/wifi/tether/WifiTetherPreferenceController.java

@@ -140,6 +140,8 @@ public class WifiTetherPreferenceController extends AbstractPreferenceController
     private void handleWifiApStateChanged(int state, int reason) {
         switch (state) {
             case WifiManager.WIFI_AP_STATE_ENABLING:
+                mPreference.setSummary(R.string.wifi_tether_starting);
+                break;
             case WifiManager.WIFI_AP_STATE_ENABLED:
                 /**
                  * Summary on enable is handled by tether