Documents management mode; API adjustment.

Create documents manage mode to support Downloads and transient
storage devices.  Locks user into requested backend root, and forces
file sizes on and sorting by last modified.

Separate API constants for Documents versus Roots, and give concrete
MIME types for roots.

Treat null sizes as unknown.  Documents are always enabled in list
so that divider is drawn.  Mark external storage file as writable.

Bug: 10329983, 10332993, 10332952
Change-Id: I05f4fdf5b04041a38e1ba7fb30202a3b0c615bf6