Report update frequency to GetCM

To reduce load on GetCM and the mirrors during releases, it would be
nice to stagger automatic update checks.  Due to privacy conerns, we
probably want to avoid sending a device id of any kind.  Instead, we can
pass the update frequency and throttle reporting of new builds to
clients that are set to check for updates at a high frequency, such as
twice daily.

This change adds an "updateFrequency" query parameter to accomplish
this.

Change-Id: Ia7c8518f80908bf6b2d1a7f4f9dd11a7c423b3d3