ART: add hook for framework to set SELinux context

Adds a new zygote hook for system server, nativePostForkSystemServer,
so the process can transition between the system_server_startup and
system_server SELinux domains.

Memory resources for the JIT are allocated in the hook as setting the
SELinux domain with setcon() requires that the process is still single
threaded.

Bug: 66095511
Test: device boots
Test: art/test.py --host --64
Change-Id: Ic840634c5c59906b8d344c2edffafeb9b13a409f