Upload chunk fetchers create background tasks even with background sessions

[mwyman]

The chunk fetchers using a background session still create background task identifiers. Because the upload is moved out of process, when the app is backgrounded the task identifier may not be released before the app runs out of time.

When GTMSessionUploadFetcher has .useBackgroundSessions=YES, GTMSessionFetcher .useBackgroundSession is not being set on chunk fetchers explicitly but being implied due to having a session identifier (sets .usingBackgroundSession=YES).

The test for whether to create a background task identifier only checks .useBackgroundSession. Should check .usingBackgroundSession instead.