Use the correct app_id when pre-authorizing remote-desktop portal access.

The portal's isAppMegaAuthorized() looks up the caller's specific app_id in
the PermissionStore.  An empty string only matches apps the portal cannot
identify; it is not a wildcard.  Since the input handler is launched via
KIO::CommandLauncherJob with a desktopName, the portal resolves it to the
desktop file ID, so the empty-string entry never matches.

--- a/inputhandler/xdgremotedesktopsystem.cpp
+++ b/inputhandler/xdgremotedesktopsystem.cpp
@@ -66,7 +67,7 @@
     QDBusReply<void> reply = permissionStore.call(QStringLiteral("SetPermission"),
                                                   QStringLiteral("kde-authorized"), // table
                                                   true, // create table if not exists
                                                   QStringLiteral("remote-desktop"), // id
-                                                  QLatin1String(""), // app (empty for host applications)
+                                                  QStringLiteral("org.kde.plasma.bigscreen.inputhandler"),
                                                   QStringList{QStringLiteral("yes")}); // permissions