[native] Avoid importing Alert directly from react-native

Summary:
Our custom Alert in native/utils/alert.js just wraps the default one with a userInterfaceStyle call based on the current user theme.

Depends on D12123

Test Plan:
I made sure the app runs with these. I had to remove the usage of our custom Alert from redux-setup.js because it led to this error:

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12124