MtpServer changes for MtpStorageManager

MoveObject and DeleteObject require
begin() and end() methods in order to
maintain database consistency. delete() now
has to return errors.

Allow sendObject after sendObjectInfo since
the spec allows this.

Test: See tests for main MtpStorageManager change
Bug: 63143623
Change-Id: Ied105e884cafd36e861521dcc59740e23b330f5f