Start fleshing out new storage APIs.

Introduces new DocumentsContract which storage backends must
implement.  Backends surface a simple directory-like organizational
structure that enables a document to appear at multiple locations in
that hierarchy.  Querying a document or the contents of a directory
will return a Cursor populated with DocumentColumns, which includes
simple metadata.

Adds new OPEN_DOC and CREATE_DOC Intents, and permission to protect
storage backends.

Change-Id: Ib4984bc980182b2cedbe552908e5be94604ef085