Rename offer() to add(), remove/hide some APIs.

Change MatrixCursor offer() to add() based on review feedback.  Remove
some now-unused XML metadata.  Hide document creation, since it's not
supported in initial version.

Bug: 10577809, 10461706, 10678563
Change-Id: I7b3f4c6899c28f633471ef816bee22bf032aa0d2
9 files changed