Add initial implementation of date-time-settings MD settings page.
Also, I fixed two issues in a11y page:
1. settings. -> settings.settings. (b/c the names were already prefixed with "settings.")
2. Correctly concat the list of prefs to request via chrome.send() (this will go away later anyway when we switch to settingsPrivate API.)
BUG=
464898
Committed:
https://crrev.com/c3a23f04cab52b8e5ba02a724237fcb65bb0e354
Cr-Commit-Position: refs/heads/master@{#324163}