Don't return query from db API

The cell_update() method of the sqlalchemy db API returned a Query when
it really intended to return a Cell.  Fix this and tweak the db API test
so that it verifies that the result of this method is a Cell.

Change-Id: I7ab7ea19f00ac2e8c2c73afe8a14f331940b4580
Closes-bug: #1198796
Merge "Don't return query from db API"