Export *CLASSPATH variables after apexes are configured.

For now, export the exact same values, on `post-fs-data` instead of on
`early-init` to soak the change. As a follow up, the actual values will
be generated by a new oneshot service.

See go/updatable-classpath for more details.

Bug: 180105615
Test: manual - device boots
Change-Id: I5f6826a0f87a5e01233e876d820e581feb555bca