Tightening visibility and removing a public_dep.

This CL aligns the visibility and the dependency between the _objc
and the _generic targets to the standard decided in:
https://codereview.webrtc.org/3001623003.

BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/3010133002
Cr-Original-Commit-Position: refs/heads/master@{#19687}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9c296b3490a40491455061209fdf409ce5214b19