Rename media constraints model and store.

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}
diff --git a/webrtc/examples/BUILD.gn b/webrtc/examples/BUILD.gn
index 8a71895..76adf29 100644
--- a/webrtc/examples/BUILD.gn
+++ b/webrtc/examples/BUILD.gn
@@ -241,10 +241,11 @@
         "objc/AppRTCMobile/ios/ARDMainView.m",
         "objc/AppRTCMobile/ios/ARDMainViewController.h",
         "objc/AppRTCMobile/ios/ARDMainViewController.m",
-        "objc/AppRTCMobile/ios/ARDMediaConstraintsModel.h",
-        "objc/AppRTCMobile/ios/ARDMediaConstraintsModel.m",
-        "objc/AppRTCMobile/ios/ARDMediaConstraintsSettingsStore.h",
-        "objc/AppRTCMobile/ios/ARDMediaConstraintsSettingsStore.m",
+        "objc/AppRTCMobile/ios/ARDSettingsModel+Private.h",
+        "objc/AppRTCMobile/ios/ARDSettingsModel.h",
+        "objc/AppRTCMobile/ios/ARDSettingsModel.m",
+        "objc/AppRTCMobile/ios/ARDSettingsStore.h",
+        "objc/AppRTCMobile/ios/ARDSettingsStore.m",
         "objc/AppRTCMobile/ios/ARDSettingsViewController.h",
         "objc/AppRTCMobile/ios/ARDSettingsViewController.m",
         "objc/AppRTCMobile/ios/ARDStatsView.h",