[native] Rename accountSelections to accountSelection

Summary:
Really minor diff. This matches the name of the type introduced in the last diff (D8088) a little better, and I think is a clearer name for this.

Depneds on D8088

Test Plan: Flow

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8089