p2p: Close the channel when onStop is called.

P2P dialog is moved to an indenpendent activity, it will push Settings
to background and trigger onPause(). As P2P dialog is a Dialog Activity,
it won't trigger onStop, but SoftAp and NAN activity will. Moving
channel closing to onStop to adapt to new P2P dialog design.

Bug: 219406778
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2pSettingsTest
      create a p2p connection between 2 devices with this build.
Change-Id: I2fab5aa1021ec1a993f811c7310079db9d7f03c0

src/com/android/settings/wifi/p2p/WifiP2pSettings.java

@@ -375,13 +375,20 @@ public class WifiP2pSettings extends DashboardFragment
         super.onPause();
         if (mWifiP2pManager != null && mChannel != null) {
             mWifiP2pManager.stopPeerDiscovery(mChannel, null);
+        }
+        getActivity().unregisterReceiver(mReceiver);
+    }
+
+    @Override
+    public void onStop() {
+        super.onStop();
+        if (mWifiP2pManager != null && mChannel != null) {
             if (!mLastGroupFormed) {
                 // Close the channel when p2p doesn't connected.
                 mChannel.close();
                 mChannel = null;
             }
         }
-        getActivity().unregisterReceiver(mReceiver);
     }
 
     @Override