Add OpenCowReader interface method to dynamic partition control am: c82511cd8f am: ebc8de1a1f am: 9e9d7eed1c

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1491178

Change-Id: Iaf224e520461b8ab1ebb7476e5354d3ff4a4172f