Refactoring of DocumentsContract.

Combines related columns and constants onto the same class so they
are easier to discover.  Move back to surfacing roots with columns
so they are consistent with documents.

Advanced roots are represented with a flag instead of distinct
types.  Flags to indicate supporting of well-known media types,
instead of arbitrary an MIME filter.  Reintroduce well-formed rootId
to support recents.

Always use the expanded version of "documents" in constants, methods,
and argument names.

Refactor DocumentProvider method names to clearly distinguish if
a single item or multiple could be returned, and of which type.  Add
documentation to clearly define which methods have already been
overridden.

Bug: 10567506, 10567557
Change-Id: I981f26ab82f2b520a19aa1ce66f659de50d7fac0