Add ro.serialno sysprop to JSON output

To help with error reporting on the RKP servers in the future it
will be helpful to be able to address CSRs by serial number when
possible.

Bug: 264302050
Test: libkeymint_remote_prov_support_test and sent JSON to server
Change-Id: I2808441c200d0679e618580abc464cd3c71c220e