Add update_engine prefs dir to bugreport

During OTA update, update_engine will cache some states, such as buffer
offset, to prefs dir. These states are used to pause/resume an OTA
update. Add these prefs to bugreport for debugging purposes.


Test: th
Bug: 245549982

Change-Id: I9b36f418eb26c76bc9538a167a0bec7c253dfc49