Exec_start derive_classpath on post-fs-data.

The service parses and merges configs from multiple partitions, defines
*CLASSPATH environ variables' values and writes them to file, for
init to export.

See go/updatable-classpath for more details.

Bug: 180105615
Test: manual
Change-Id: I60c539a8fef4d690f47704e896f67949ec49db60