media-keys: Avoid a critical warning

Commit 042f75b21857208680d3c3751dc5a9e755c8418a introduced a
setting of type uint, but used g_settings_get_int() to access it.
This yields:

(gnome-settings-daemon:30705): GLib-CRITICAL **: g_variant_get_int32: \
assertion 'g_variant_is_of_type (value, G_VARIANT_TYPE_INT32)' failed

whenever one uses the C-A-S-r shortcut. Use g_settings_get_uint()
instead, to avoid this.