Import downloader code to AOSP. am: c0449c8753 am: 3ebc998629 am: f1a7677242 am: d51fc2721f

Original change: https://android-review.googlesource.com/c/platform/external/downloader/+/1591311

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ie4af46f266fa1539ae9f9a711e7b870df48c268c