DocumentsUI handles GET_CONTENT; hinting, errors.

Document browser now takes over all GET_CONTENT requests that request
openable Uris. It shows both storage backends and includes other apps
that respond to GET_CONTENT. Only grants transient read permissions.

Better guarding against throwing storage backends. Send sort order
and local-only hinting to backends.

Require that OPEN/CREATE_DOC users include openable category.

Bug: 10330112, 10329976, 10340741, 10331689, 10329971
Change-Id: Ieb8768a6d71201816046f4a4c48832061a313c28