android: Add select all button to action mode

Thanks to Jason for suggesting the not-clinically-insane
method to go about this.

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

app/src/main/java/com/wireguard/android/fragment/TunnelListFragment.java

@@ -305,6 +305,10 @@ public class TunnelListFragment extends BaseFragment {
                             .whenComplete(TunnelListFragment.this::onTunnelDeletionFinished);
                     mode.finish();
                     return true;
+                case R.id.menu_action_select_all:
+                    for (int i = 0; i < tunnelList.getAdapter().getCount(); ++i)
+                        tunnelList.setItemChecked(i, true);
+                    return true;
                 default:
                     return false;
             }