Make enable/provisioning of the backup service a two-step process

This CL adds the concept of 'provisioned' to the backup manager.  No backups
will be scheduled until the user has indicated that backups are to be enabled
*and* has clicked all the way through the setup wizard.

When the user first turns on the backup system, the delay before the initial
backup pass is different from the periodic backup interval.  Currently that
initial delay is 12 hours.  The intent here is to guess at a less-active time
for performing that first backup pass.

NOTE: currently the backup service defaults to 'provisioned'.  Once the real
code goes live in Setup Wizard, this will be changed to default to
not-provisioned until the user has confirmed all the relevant UI.