Replace -classpath with --class-loader-context for preopt

Use --class-loader-context to pass the context to dex2oat instead of using
runtime arguments.

In preparation to simplify dex2oat handling of contexts.

Bug: 37966546
Bug: 38138251
Test: make & check that the prebuilds have '&' as their classpath key

(cherry picked from commit 965f8aec)

Change-Id: Id027e1fcbd69ac5e338214a03ae44222988544f3