fs_mgr: retrofit VAB update after A/B calc COW space.

When applying retrofit VAB update on a regular A/B device,
if device is at version Android R, update_engine may run
a virtual A/B update directly. In this case, partitions
with suffix B in slot 0 (current slot) should not be treated
as unusable space by CoW.

Fixes: 143323939
Test: libsnapshot_test

Change-Id: Ic845374e519885d21e021e97cb32fab9f5d56a63