[native] Animate BottomSheet closed on goBackToPrompt

Summary:
goBackToPrompt gets called if the user presses the back button, or uses the Android hardware back button.

Before this diff, the BottomSheet would stay in place until it disappears when SIWEPanel unmounts. Now it will animate closed.

Test Plan: Press the back button and make sure the BottomSheet animates closed

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6019