zygote to preload public native libraries

Preload public native libraries when starting a zygote
This lets other users of libnativeloader (like dex2oat)
avoid unnecessary cold loads of libandroid.so which could
take up to a second.

Bug: http://b/27245894

(cherry picked from commit b0824979)

Change-Id: I04a640c104dba5b585a59d63270e81b6c9ad6f7e