More UX work for thumbnails, search, management.

Hide non-finished downloads from normal picker UI, but keep them
around in management mode.  Uses a Uri query parameter and a hidden
API on DocumentsProvider.

Scale thumbnails to fit viewport, always show MIME icon while waiting
on thumbnails, and crossfade between them.  Cancel thumbnail tasks
when views are recycled.

Filter directories out of search results for now.  Also leave sort
ordering from backend intact, since it's custom ranking.  Fix
SearchView interaction to dismiss properly and restore across
orientation and drawer state changes.

Hide most actions when drawer is open.  Invalidate RootInfo cache
when locale changes.  Apply sort ordering when showing recent create
directories.  Hide recent summary string when icon is enough for user
to disambiguate.

Bug: 10667184, 10665663
Change-Id: I331d3272a08c497f88dc659d9e112231cb35aa69