This change enables SELinux security enforcement on zygote
(but not zygote spawned apps).

For the zygote.te file only, this change is equivalent to reverting
the following commits:

* 50e37b93
* 77d4731e

No other changes were required.

Testing: As much as possible, I've tested that zygote properly
starts up, and that there's no problem spawning zygote or zygote
apps. There were no denials in the kernel dmesg log, and
everything appears to work correctly. It's quite
possible I've missed something. If we experience problems, I
happy to roll back this change.

Bug: 9657732
Change-Id: Id2a7adcbeebda5d1606cb13470fad6c3fcffd558