Calculate the runtime ro.build.id in ota scripts

If the build prop ro.build.id isn't set at build time, init will
set it at runtime. The logic is appending the vbmeta digest to
the ro.build.legacy.id.

Make the same change in ota scripts, so the correct build fingerprint
will be saved in the ota metadata.

Bug: 186786987
Test: generate an OTA, check the metadata
Change-Id: I278f59c41c1f98d4cbea749e5d9e4eaf7a6b9565