DocumentsUI: Add new private intent action to select copy destination.

The CL adds INTENT_ACTION_OPEN_COPY_DESTINATION, and implements the behavior to
respond the intent action.

The directory picker:
 * filter out read only roots
 * return document URI instead of document tree URI
for the intent action.

BUG=b:20146034
Change-Id: I96c565100c3cdaa57cc7abd8cf6d895d148941a7