Add a neverallow rule (CTS test + compile time assertion) blocking
system_server from executing files outside of a few select file
types.

In general, it's dangerous to fork()/exec() from within a multi-threaded
program. See
https://www.linuxprogrammingblog.com/threads-and-fork-think-twice-before-using-them
This change helps discourage the introduction of new execs.

Bug: 28035297
Change-Id: Idac824308183fa2cef75f17159dae14447290e5b