Fix deserialization issue caused by ManualPatchsetCreated class move

xstream compatibility alias was defined when the class was moved from
the gerrit-event to the gerrit-trigger but TriggerContextConverter is
not using xstream mapper. Modify TriggerContextConverter to use xstream
mapper instead of java class forName method.

Change-Id: I3280257e1740f5c0612400de4ccd0e862ace1250