Use optional assignments for some virtual_ab properties

When using the normal assignment, it is impossible to override
PRODUCT_VENDOR_PROPERTIES in a device config because the property
will have two different values and the build system won't know
which one to choose.

This changes the io_uring and compression.xor assignments to
optional because low memory devices may want to override these to
relieve memory pressure when applying an OTA.

Test: Manual, props remain true by default but can now be changed
Bug: 341085461
Change-Id: I3a49fb07b7a621c68a34aceeba1461970d2fba6a