GitLab

- Implement GetWebsiteSettingLastModified for the
NotificationChannelsAndroidProvider by storing channel creation
timestamp within the channel IDs.

- This also has the beneficial side effect that each time a channel
is created in response to permission grant, it will have a unique ID,
so that channels deleted when site data is cleared will not be
restored the next time permission is granted/blocked.

- Note that NCAP.GetWebsiteSettingLastModified is currently only
called from tests, but in a future patch it will be pulled up to a
new provider interface and called by the HostContentSettingsMap
where it currently only calls the Pref Provider. 

Bug: 700377
Change-Id: I5363ae2baa7e0c92bb7768361d459cd024622b32
Reviewed-on: https://chromium-review.googlesource.com/567153
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488198}