search: Use <bundle> ID to determine flatpak app ID

The <bundle> element in the appstream data unambiguously provides the
full four-part flatpak ref, so use it to determine the app ID. But fall
back to using the <id> element, since that is required to be present.

(cherry picked from commit 39de0ef280a98f67d639444cc6ea3bcfa61c0eec)