LATEST(expr, maxBytes) doesn't work on columns ingested as stringLast.

### Description

I have ingested few columns using stringLast aggregator, which stores the data as pair of time and value. I got to know that the idea is to use the same aggregator on both ingestion and query sides. But the LATEST(expr, maxBytes) doesn't work on columns stored as stringLast.

### Affected Version

Version - 0.17.0

[Slack thread](https://the-asf.slack.com/archives/CJ8D1JTB8/p1587594263292900)

No source files or tests are named. Start by reproducing the failure in Druid 0.17.0 with columns ingested using the stringLast aggregator and queried with LATEST(expr, maxBytes), then trace the relevant ingestion and query aggregation paths. Done means LATEST works correctly for columns stored as stringLast, with regression coverage for the reported case.

Written by the indexing model from the issue text.