Map correct My files path for crostini path sharing

Map paths into the container as /mnt/chromeos/MyFiles/Downloads
rather than /mnt/chromeos/Downloads to prepare for coming changes
with MyFiles.

Made all crostini mappings to use CamelCase for dir names.

Bug: 878324
Change-Id: I7c1635dab406242d43d95ee7d44730ee11f6758f
Reviewed-on: https://chromium-review.googlesource.com/c/1309555
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604526}
4 files changed