add a flag to enable/disable download interception on android

Currently some of the download are passed to Android DownloadManager.
We might change that behavior in the future.
This CL adds a flag to allow us to toggle that behavior.
Chrome flag and finch flag "EnableDownloadInterception" are added to allow locally/remotely set/unset the flag

BUG=545640

Review URL: https://codereview.chromium.org/1692693003

Cr-Commit-Position: refs/heads/master@{#377724}
6 files changed