Reland "Change to default directory when removable entry list is unmounted."

This is a reland of 499a46c274415cbae3dc022fd13d4ee6b2c6bddb

Original change's description:
> Change to default directory when removable entry list is unmounted.
>
> Fixes bug where file list and breadcrumbs don't updated after removable
> entry lists are unmounted.
> Added unmount for removable test volumes, to test an unmount event.
>
> Bug: 918795
> Change-Id: I13ea71b6db9dff5ac2e59982eef7f6e34457fe31
> Reviewed-on: https://chromium-review.googlesource.com/c/1465962
> Commit-Queue: Alli Murray <alliemurray@google.com>
> Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#632481}

Bug: 918795
Change-Id: Idee5fcf4f17aa672468ac9ba39f5168d20a126bf
Tbr: noel@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1474961
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632995}
9 files changed