Storage roots in fragment, sectioned.

Move storage roots into a fragment, since it's not a drawer on
tablets.  Cluster and sort roots when displaying.  SectionedListAdapter
to make clustered roots easier to manage.  Add docs for root types.

Move roots cache into separate class to make it easier to share.

Change-Id: Ia0b92eade059e816324641f600c08026c0e268c9