Don't set the build id if we need to append the digest

Background in http://go/compatible-build-fingerprint. If we want
to append unique vbmeta digest to build id, we cannot setup the
prop value at build time. Instead, set the old value as
ro.build.legacy.id; and let init set ro.build.id at runtime.

Bug: 186786987
Test: build a target file with the flag on
Change-Id: Ie139725bb7e5c65bd3f28f43b9975ba48ee10354