Add partition writer class. am: 9bd519d4f5 am: 8a188256ba am: 88e2d6d48c am: 95169d81ac

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1435571

Change-Id: Ifdbbbf920f3c0600c2975b01650bc9f228ac78eb