DO NOT MERGE: Listen to file changes on Downloads dir.

And notify clients of Downloads when files on disk.
Cherrypicked from: e398c096892fe8aac0c46374aec6bbe016c41016 and
    2b011b97f96463c1f4a4f6b69c5bbe18ec0fc572 (followup changes
    due to autocommit commiting changes before completing nits).

Bug: 28430547
Change-Id: I42a42bfb5e585918fe6b384a862699445afefcbc
1 file changed