Copy the updates for uncrypt using a Service

Same as change I488018d01c0baf74660362a384e53bfe5d85de2b
("Use a service to export the updates"). While at it, move the code
out of UpdaterService to better separate the code that deal with AB
updates and regular updates.

Change-Id: I2bc3e78e80f5e63c57303cbbcdc0353dbab0f67f