Move the stable code from version_defaults.mk to version_util.mk.

With version_defaults.mk containing only simple assignments, it can be
automatically converted to Starlark.
This is a cherrypick of ag/15768054

Bug: 198995713
Test: treehugger
Change-Id: I4798fb69c639e6d52d275a5b793fa0c8ba876202
Merged-In: I4798fb69c639e6d52d275a5b793fa0c8ba876202