The new version of Flow is stricter with types, and exposed several errors in our typing of navigators and screens.
Some of these are pretty bad, but the good news is that Flow should make sure we don't make the same mistakes again going forward.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D9991