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: 38138251
Test: make & check that the prebuilds have '&' as their classpath key
Change-Id: Iad2fc4364173f59f41886556222ec69f87a0bd52