Prepare Soong for switch to stable core/platform API.

This ensures that `m nothing` succeeds even if java/config/config.go
is changed to use `stable.core.platform.api.stubs` and
`stable-core-platform-api-stubs-system-modules` instead of the legacy
versions. (The change to stable cannot be made yet because real
targets depend on APIs only in legacy, but it is useful for the build
system to be ready for this change.)

Bug: 157640067
Test: m nothing (with and without local change mentioned above)
Change-Id: Id7562d8c592551fd0ed2b9849bcf6eb707c2a1d0