Initial implementation of OPEN_EXTERNAL_DIRECTORY.

It opens a an alert dialog that tells user application X is requesting
access to folder Y on volume Z; if user accepts, it returns the proper
URI, otherwise it returns RESULT_CANCELED.

The requested path must satisfy the following requirements:

1.It's directory (getName()) must be one of the pre-definied folders on
  Environment.
2.It's root (getParent()) must be a valid external storage root.

In order to satisfy #2, DocumentsUI calls the ExternalStorageProvider,
through a custom call() implementation.

BUG:  23011462
Change-Id: I8e491e82ae320df88f7b8923286c4c1ad050ae80