CrOS FilesApp menu Share with Linux only for downloads subfolders

Only show the 'Share with Linux' menu item for subfolders of
downloads.

Added getMetadata method to Linux files FakeEntry which is called
when 'My files' is selected and 'Downloads' and 'Linux files' are shown
in right hand files list.

Tests verify that 'Share with Linux' menu is not shown for:
* Downloads root
* Folders in crostini
* Folders in drive

Bug: 878230
Tbr: lucmult@chromium.org
Change-Id: I4afcaaa667d6cb7e5ec10ea3659819a937a2bedf
Reviewed-on: https://chromium-review.googlesource.com/1209006
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589126}
7 files changed