[native] Introduce derivedDimensionsInfoSelector for safeAreaHeight

Summary:
Following up on conversation here. I had remove native/selectors/dimension-selectors.js earlier, thinking that we could just use DimensionsInfo directly.

But I think there is a valid point to be made that we are copy-pasting the safeAreaHeight calculation in too many places. So I'm reintroducing a selector here to cache the derived data.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D55