DocumentsUI: Filter out downloads root if the source documents include directory.

This is the short term solution for the downloads root, which does not support
directory creation. For the long term solution, we need to add a flag to Root
model to check if the root support directory creation or not.

BUG=20186663

Change-Id: I15c4795892f38e3f968776f732a6f2160ae775fc
4 files changed