[native] Extract useKeyboardHeight from LoggedOutModal

Summary:
We'll want to use this (with slight modifications) in the next diff, where we convert LoggedOutModal to Reanimated 2 syntax.

Depends on D12198

Test Plan:
I tested this diff stack by playing around with LoggedOutModal on my iOS simulator in my local environment:

1. I navigated to LogInPanel and back
2. I navigated to LegacyRegisterPanel and back
3. I navigated to FullscreenSIWEPanel and back
4. I navigated to QRCodeSignInNavigator and back
5. I navigated to Registration and back

Reviewers: inka

Reviewed By: inka

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12207