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
9ecfee03fa