Moved download task scheduler out of background_service

Since both download service and auto-resumptions depend on task scheduler
and task manager, it would be ideal to move them out to a common directory.
This CL moves them out to //components/download/public/task

Bug: 920706
Change-Id: I81b06d9d4e4d577fed48ee8e9c57600ee7b4a793
Reviewed-on: https://chromium-review.googlesource.com/c/1405544
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621826}
24 files changed