[landing] rename asset to imageAsset

Summary:
Small rename diff. In the future I plan to bring back asset but typed differently. This diff is to prepare for that moment

Depends on D8054

Test Plan: flow and no obvious regressions

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8055