bind android.bg in system_server to small cores

android.bg in system_server primarily handles PSS collection, which is
not performance critical. Accordingly, bind it to little cores.

However, because threads in zygote-derived applications are only in a
single cpuset at a time, this also means the scheduling policy for
system_servershould never be reset by applyOomAdjLocked(). The only
time this happens is when system_server's ProcessRecord is first
created, and its scheduler configuration is already controlled by
init.rc. Configure the right defaults for system_server's scheduling
group so it's never modified from applyOomAdjLocked().

Test: android.bg is in background, rest of system_server is in fg
bug 70859548

Change-Id: Id8582e6d493c6b2d3390fdc45ac2396390744f06