Enable gwp-asan in system_server.

Enables gwp-asan in system_server with a small probability, same as any
native process or system app.

Bug: 149991821
Test: kill system_server in a loop - find "GWP" in /proc/$PID/maps
      eventually
Change-Id: I3900f5d277d28987883a918d62fe9390c780ac30