[native] Rename FullscreenSIWEPanel onClose prop to goBackToPrompt

Summary:
After writing the diff description for D7928, I figured this change would make the code a bit more readable.

Depends on D7928

Test Plan: Flow

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7929