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
NOTRY=True

Review-Url: https://codereview.webrtc.org/3012753003
Cr-Original-Commit-Position: refs/heads/master@{#19668}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: edb8429ba3df2f359813498e63bc269fe495f3f1