Fixes for avb flags in soong-generated partitions

The logic for what variables control what aspect of AVB is pretty
complicated, this brings make and soong closer together.

Bug: 381120092
Test: m out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_system_image/android_common/prop, diff with make's prop file
Change-Id: I6ada90262f4578d120c35b7f8d38c06c2b1d9c27