Bug 26635: (follow-up) Rename av_expand to strings

Bug 26635: (follow-up) Rename av_expand to strings

This patch updates the `av_expand` occurences to `strings` to match the
change of the return structure of `_strings`.

We replace `+av_expand` in the headers with `+strings`, the expected
object method name from `api_av_mapping` to `api_strings_mapping` and
the internal hash key from `av_expand` to `strings`.

Test plan
1) Run the included unit tests.. all should still pass.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>