Change PW to track Nearby/Resolved URLs separately

Previously we would keep a cached list of nearby URLs (as found via
Bluetooth scans), and not store the list of URLs which were properly
resolved via the Physical Web Service.  Instead we would make network
calls each time for the full set.

This change introduces a separate cache for known resolved URLs,
enabling a few improvements:
- PWS resolves are now sent 1 URL at a time, which is more efficient and
allows out-of-order replies when we find multiple beacons at once.
- We can create a notification right away if we find a beacon which we
have ever resolved in the past.
- When the notification is clicked, we only display the known resolved
URLs, showing us results faster.

BUG=529962

Review URL: https://codereview.chromium.org/1483653002

Cr-Commit-Position: refs/heads/master@{#364197}

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