Remove the usage of Downloads.* constants.

Replaced them with local constants as it is only for
internal use.

Test: basic sanity
Bug: 140908357
Merged-in: Icebabc4d16c2e6d5b6e2fb311fee697e8acfa285
Change-Id: Icebabc4d16c2e6d5b6e2fb311fee697e8acfa285
(cherry picked from commit 113d4543)