Build: Do not treat org.apache.http.legacy.boot as boot classpath

For compilation purposes, treat org.apache.http.legacy.boot as
a regular java library, so it gets standalone-preopted.

This is a workaround until the module is properly removed from
the boot classpath, at which point it should be reverted.

Bug: 18027885
Bug: 72167111
Test: m
Test: manual
Change-Id: Ifadcd1d1e754ce215b96369623ea154d3c3321eb