[Files app] Force cross-filesystem move between My files and Downloads

Downloads being a bind mount into My files, a local move is not possible
between these locations, and calling Entry.moveTo results in a recursive
copy + delete, without progress callback. My files and Downloads should
be seen as different filesystems.

Test: Manually tested
Test: browser_tests --gtest_filter=*FileOperationManagerTest
Bug: 1200251, 959083
Change-Id: I08d19a68fef2aaec907990408abcdcab47c46ca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3012437
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900830}