listaddressgroupings reports a value that aggregates all assets

This is a statistic that doesn't make any sense. This should be changed to a map from asset id to amount.

Start at the listaddressgroupings RPC entry point and trace how its result aggregates asset amounts. Look for related wallet or RPC tests and determine the expected per-asset representation. Done means the result maps each asset ID to its amount rather than reporting one aggregate value.

Written by the indexing model from the issue text.