Fix naming issue when renaming files inside My files

Change DirectoryModel to "unwrap" VolumeEntry when it's selected via
file list. This fixes the issue where after selecting a VolumeEntry any
operation in the private API would fail because it wouldn't be able to
send a VolumeEntry to private API, because it requires a real Entry.

This fixes the user issue of trying to rename and always getting "Use
shorter name" error, even with a short name.

Update FakeEntry to be an implementation of FilesAppDirEntry, since all
users were using as directory, also update some functions to typing to
use FilesAppDirEntry instead of FakeEntry which is more correct.

Test: browser_tests --gtest_filter="*/myFilesFolderRename"
Bug: 889636
Change-Id: Ieb75ffbb811a91e4742fb7a49a3badfd129376ad
Reviewed-on: https://chromium-review.googlesource.com/c/1276067
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599136}
9 files changed