update_verifier: Support verifying product partition.

We have added the support for building /product partition in build
system (the CL in [1]), where /product is an optional partition that
contains system files. This CL adds the matching support if /product
needs to be verified during A/B OTA (i.e. listed in care_map file).

[1]: commit b7735d81054002961b681f4bdf296d4de2701135,
https://android-review.googlesource.com/c/platform/build/+/598454

Bug: 63974895
Test: Run update_verifier test on walleye.
Change-Id: Ia1c35e9583b8e66c98a4495b1f81a5ea7e65036f
(cherry picked from commit ec2e8c6c)