Refactor extent writer to take filedescriptor in constructor am: 4d22ca2ab6 am: ac10c6c57a

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1581806

Change-Id: Ibd6bd0d9e702f8b73fbac3ae8b3d97f8f1e55e08