Make RecentsLoader load on a per-authority basis instead of per-root.

RecentsLoader currently spins up a RecentsTask for each individual root,
and we have a threshold of how many tasks can be ran in parallel to
prevent too many Providers getting spun up. However, it's possible that
multiple roots share the same authority/provider. Changing the Loader
behavior so it spins up one Loader per authority, as opposed to per
root.

Bug: 31682079
Change-Id: I64a66b4d4573d55267c45e16ee4d648d239c1a07