[backup] Extract function to query DDB index

Summary: Noticed that the same query is used in two places, and I'm going to introduce third one.

Test Plan: Created two backups. Made sure that behavior hasn't been changed: the latest is retrieved, and the old is removed.

Reviewers: kamil, marcin, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12970