snapshot_proto add build configuration variables

We are adding low memory device configurables. Post OTA reboot, these
values need to be read during 1st stage init and set. Since .ro props
aren't available at this stage, we need to flush these configurables to
snapshot_protos that lives under /Metadata

Bug: 332255580
Test: th
Change-Id: Iff84c0dfe9c6931ea13165b380e11cee6343ce91