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