DefaultUsageUploadService: refactor a little

In preparation for the next commit, of course.
DefaultUsageUploadService: flush usage stats

Previously, the stats weren't being flushed each time they were
processed, which would result in erroneous accumulation of object
counts being repeatedly uploaded.
Add an options dialog for privacy settings

For privacy reasons, there must be an option to opt out
of the anonymous usage statistics data collection.
POM: bump to latest pom-imagej: 2.39