Because the new API doesn't use constraints, refactors settings to instead have a
video resolution setting.
BUG=webrtc:7177
Review-Url: https://codereview.webrtc.org/2778163005
Cr-Commit-Position: refs/heads/master@{#17545}
Moves settings model and related classes to code common for both iOS
and Mac.
BUG=webrtc:7177,webrtc:6494
Review-Url: https://codereview.webrtc.org/2770113004
Cr-Commit-Position: refs/heads/master@{#17408}