Fix activity leak on Android Q

This workaround was discussed at https://twitter.com/Piwai/status/1169274622614704129 after
Google had closed the issuetracker with a WONTFIX at https://issuetracker.google.com/issues/139738913.

The situation has since changed with Google promising a fix on October's ASB but since we can't really
know, patch this ourselves for the timebeing.

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

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

@@ -53,7 +53,16 @@ public class MainActivity extends BaseActivity
             setSelectedTunnel(null);
             return;
         }
-        super.onBackPressed();
+        if (isTaskRoot()) {
+            // @{link TunnelDetailFragment} is in foreground
+            if (backStackEntries == 2) {
+                getSupportFragmentManager().popBackStack();
+            } else if (backStackEntries == 0) {
+                finishAfterTransition();
+            }
+        } else {
+            super.onBackPressed();
+        }
     }
 
     @Override public void onBackStackChanged() {