Change only_use_system_oat_files_ logic.

If we're creating a process which is a zygote, set it to true. Change it
to false on non-system-server fork.

This makes sure that zygote doesn't load oat files from /data, which
would then make system server abort at startup.

Test: 677-fsi, 677-fsi2
Bug: 198531508
Change-Id: I367fcf79f2e60a32457829ddc88b680fc2d00099