Download auto-resumptions : Moved network listeners to a separate target

Since we are planning to reuse the network change listeners in regular
download auto-resumptions, it makes sense to move them to a separate target
outside of background service.

Bug: 910874
Change-Id: Ie85b0c3bbc23b765b9b3a0cc8a21b4bebe86a877
Reviewed-on: https://chromium-review.googlesource.com/c/1353957
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613794}