Attribution is attached to bucket usage, but that's more granular than
necessary for the attribution report. This change iterates over the
bucket attributions, parses the user agent, converts the first entry
to lower case, and uses that as the key to a map which holds the
attribution totals for each unique user agent.
Change-Id: Ib2962ba0f57daa8a7298f11fcb1ac44a8bb97875