Move partition writer to a separate file am: 50bac6567e am: 263ad13e49 am: 6298fa3718 am: 92bd1bbd9d

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1441391

Change-Id: Ief79ec72f1ae483a97220083353c5f853ea828c0