MainActivity: Fix backstack bug exposed by fragment 1.2.2

We've been relying on implicit backstack changes to handle removing the detail fragment for a while which is now gone so let's do this properly like we should

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/java/com/wireguard/android/activity/MainActivity.java

@@ -50,6 +50,7 @@ public class MainActivity extends BaseActivity
         }
         // Deselect the current tunnel on navigating back from the detail pane to the one-pane list.
         if (!isTwoPaneLayout && backStackEntries == 1) {
+            getSupportFragmentManager().popBackStack();
             setSelectedTunnel(null);
             return;
         }