system_server: neverallow dex2oat exec

system_server should never be executing dex2oat. This is either
a bug (for example, bug 16317188), or represents an attempt by
system server to dynamically load a dex file, something we don't
want to allow.

This change adds a compile time assertion which will detect
if an allow rule granting this access is ever added.
No new rules are added or deleted as a result of this change.
This neverallow rule is automatically enforced via CTS.

Bug: 16317188
Change-Id: Id783e05d9f48d48642dbb89d9c78be4aae8af70c