Move search to roots; Documents root; hide empty.

Documents searches now happen root-wide, instead of only under a
subdirectory.  Updates abstract class and flags to match.  Add flag
for a root to indicate it's empty, and hide empty roots in UI unless
creating.

Define "Documents" public directory and storage backend to contain
files.

Bug: 10712057, 10710865, 10710758
Change-Id: I8716367568969f9cb1d83927b2bf5a7013809350