Add FeatureFlag for Virtual AB Compression am: da1b3145df am: be289f6d90 am: 51746ddfe6 am: bca4890101

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1437991

Change-Id: If39e18e77f1e6311e7a7e5a51a381a22a3803ca7