Add new Intent API for associating a ClipData with an Intent.

Allows applications to propagate multiple URI grants through an
Intent.

Later on, we should probably redefine the share actions to be
based on this ClipData with the old extras-based approach only
there for compatibility.  Even if we don't do that, though, this
allows you to do a multi-select share that grants multiple URI
permissions by stuffing the URIs in a ClipData.

Also add some documentation in various places telling people how
they can grant URI permissions.

Change-Id: Id4ba8e72c11caf7e1f1f438cb7af058d1586a37c