External storage provider, document picker UI.

Continuing to flesh out storage backends by adding an external
storage document backend.  Still rough, but it can traverse files
and directories.

Early pass at OPEN/CREATE_DOC picker UI, which offers to traverse
any known storage backends.  Supports opening subdirectories and
returning a picked file.

Change-Id: Idc3554036b3816a93d9b465ee8a620746859d2ae