[composebox] Don't override aep if given in additional params.

If aep is given in additional params, it could have been specified
alongside the match. Don't override this with the generic entrypoint
value that all surfaces use right now.

Change-Id: Iff7619f9398356798aefda6e5a23fabb7e7f947e
Bug: 446228132
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6970009
Commit-Queue: Nihar Majmudar <niharm@google.com>
Reviewed-by: Jason Hu <hujasonx@google.com>
Cr-Commit-Position: refs/heads/main@{#1518188}