Add platform in the base indexer file choosing logic.

[fsgeek]

In building the Dropbox ingester, I found that the base package is returning a file that wasn't specific to Dropbox. I think the right long-term solution is to allow specializing this interface to only return entries for a specific platform.

This is a work item to track doing this - it's not high priority, but I don't want to forget it either.

[fsgeek]

See commit a4e0cd2 for the change I made to achieve this just for Dropbox. It may justify more generality. Changing this will impact any other component using the base implementation in the IndalkeoIndexer.py file.