New roots UX, async, performance, docs.

Yet another iteration from UX on how roots should be ordered.  Since
we no longer categorize by type, remove from public API.  Updated
asset drop with new dividers.

Update public API docs to be explicit about required columns.  Hide
flags and columns that aren't required for third-party apps.

Move remainder of potentially blocking work to AsyncTasks, including
creating directories, picked root resolution, and creation of new
documents once picked.

Improve performance of layouts by removing baseline alignment and
reduce hierarchy depth.  Set alpha on ImageViews directly to avoid
offscreen rendering hit.

Limit returned recents to 45 days.  Show load in recents when still
waiting for backends.  Show empty message when no recents stacks to
create from.  Use unique key when saving recent stacks.

Bug: 10941423, 10819454, 10964412, 10960718
Change-Id: I08cf589dcda7e203acf67928f4d30322ae36ee94