Implement DocumentsProvider.createDocument and deleteDocument

Fixes AOSPA-431
This is needed to delete and create files/folders in DocumentsUI.
Some open questions are:
1. Are the methods implementented according to their description?
E.g I'm not sure whether deleteDocument should be able to delete folders, too.
2. I first didn't want to modify public API, but deleting files from within DocumentsUI somehow doesn't work - file.canWrite() always returns false
3. So is modfying the API this way OK?

Change-Id: Ie3a9d1888decaa0f08de1a5183ffd23760be15b9