Use checkdeps to ensure API headers don't include internal headers.

This CL updates the checkdeps configuration for the api/ folder in
order to explicitly avoid to #include non API headers from API headers.

In order to force a careful review of potential exceptions to this
rule, the CL also adds mbonadei@ and kwiberg@ as OWNERS of api/DEPS.

Bug: webrtc:9887
Change-Id: I0ada6f1020186b2782c7d060af36079c452ba1aa
Reviewed-on: https://webrtc-review.googlesource.com/c/106800
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25338}