Set NO_NEW_PRIVS on zygote init

When app_process/zygote starts, make sure PR_SET_NO_NEW_PRIVS is set.
This prevents zygote spawned apps from acquiring new privileges
on exec.

In particular, this allows the CTS test
android.os.cts.SecurityFeaturesTest#testNoNewPrivs() to pass if ART is set
as the default runtime.

Change-Id: I81139cda999c7b1430242561aad28f566e9b6da0