Move DownloadsManager to permanent whitelist.

DownloadProvider is now based completely on JobScheduler, and deep
inside the platform we allow foreground
downloads (FLAG_WILL_BE_FOREGROUND) to run even while the device is in
doze, so it doesn't need to be temporarily whitelisted anymore.

BUG: 29056149
Change-Id: I3658bb42aeeee5d5528f91ec990d6e1bc54257b6