allow system_server to set kernel scheduling priority

Addresses the following denial:

  avc: denied { setsched } for comm="system_server" scontext=u:r:system_server:s0 tcontext=u:r:kernel:s0 tclass=process permissive=0

It's not clear why system_server is adjusting the scheduling priority
of kernel processes (ps -Z | grep kernel). For now, allow the operation,
although this is likely a kernel bug.

Maybe fix bug 18085992.

Bug: 18085992
Change-Id: Ic10a4da63a2c392d90084eb1106bc5b42f95b855