[native] Avoid showing iCloud password manager prompt when dismissing LogInPanel

Summary:
For some reason, this prompt now appears when the LogInPanel is dismissed. It seems that switching to Reanimated 2 syntax here changed something about how the iOS password manager logic works with our code. (One big plus is that in LogInPanel, the username and password are now autocompleted together, instead of requiring two separate steps – cc @varun, who previously talked about this with me.)

Anyways: it makes sense to show this prompt when the user attempts a login, but not when the user goes back.

To avoid showing the prompt, we can clear the password TextInput before it disappears from the screen.

Depends on D12208

Test Plan:

Reviewers: inka

Reviewed By: inka

Subscribers: varun, tomek

Differential Revision: https://phab.comm.dev/D12209