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The WifiPollingPolicy controls how frequently the network location
provider will initiate Wi-Fi scans. Previously, the policy was attached
to the WifiDataProvider, which is destroyed when there are no active
geolocation API calls. Destruction of the provider causes us to lose track
of our polling policy.

The policy allows the first scan to be performed immediately and schedules
subsequent scans at regular intervals. When the provider is destroyed and
recreated rapidly it considers each scan to be "first", which allows many
Wi-Fi scans to be performed in a short window.

This change moves the policy state to a global instance so it can be saved
when the provider is destroyed and recreated. The new policy allows the
first-ever scan to be performed immediately, and subsequent scans may also
be performed immediately if enough time has lapsed since the previous scan.
In all other situations, the configured polling interval is enforced.

BUG=764954

Change-Id: I00649dde2a707127e53e0330511172fef4a43471
Reviewed-on: https://chromium-review.googlesource.com/834988
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526782}