Move non-updatable-api/ to core/api/

This achieves two things:
- it's been noted that non-updatable-api is a confusing term as it has
  to be updated every time the API changes
- the api-review logic trigers on txt files in subdirs named 'api',
  which non-updatable-api does not trigger on

I will (less urgently) try to get rid of the term "non-updatable" from
other places too, perhaps going for "non-module" or "core" as a
replacement.

Bug: 171029603
Test: m checkapi
Change-Id: I843efb4880b6caaf3872e9b0918fd710fb93d979