ARDMediaConstraintsModel -> ARDSettingsModel
ARDMediaConstraintsSettingsStore -> ARDSettingsStore
ARDMediaConstraintsModelTests -> ARDSettingsModelTests
Both classes will be used to storing other settings as well, so we need a
more appropriate naming.
BUG=webrtc:6654
Review-Url: https://codereview.webrtc.org/2479153002
Cr-Commit-Position: refs/heads/master@{#15002}