DefaultUpdateService: remove unneeded service deps

The LogService and StatusService were once used by the
DefaultUpdateService, but that logic migrated to a dedicated command:
CheckForUpdates.
CheckForUpdates: make some services optional

There is no reason to require the LogService or StatusService.
Let's just use them when they are present.
DefaultUpdateService: tweak event handler

We want to avoid a warning about an unused parameter, but the
@SuppressWarnings annotation is long and ugly. We can also avoid it by
adding a @param declaration to the javadoc, so let's do that instead.
UpdateService: add API to get a file's update site

This change adds a little bit of infrastructure to keep an internal
FilesCollection instance in the DefaultUpdateService, which is
initialized lazily. The FilesCollection is then used as needed
when relevant UpdateService methods are called.

The first such method is getUpdateSite(File), which will be needed
by the UsageUploadService when deciding which statistics are worthy
of being uploaded to the ImageJ usage server.