Handle non-existent item when calling update_item

If update_item action(s) is(are) ADD and/or PUT, a new item
will be created if expected conditions are satisfied. Otherwise
ConditionalCheckFailedException will be thrown. If
action(s) is(are) DELETE only, no operation will be performed
and a success response will be returned.

Closes-Bug: #1373627

Change-Id: Ib58d1433dd546f1472129b669f14a1b689647e1d
Merge "Handle non-existent item when calling update_item"