Use the private Download Manager APIs.

This boils down to using Downloads.Impl instead of Downloads

The APIs that were being used so far are going to disappear
to be replaced by a new set of APIs, but in order for the browser
to be able to continue using the old APIs a new copy of those APIs
was created "on the side" so that the browser doesn't need to
change much.

Bug: 2245521

Change-Id: I33c526331cce006710b1f934a4aeb64cdb95d62e
4 files changed