Bypass surface flinger permission check for calls from system.

Early during the boot, before activity manager is ready to handle
permission checks, the system needs to be able to change the display
state.  Added a hardcoded exemption for AID_SYSTEM (which already
has permission to talk to surface flinger anyhow).

Bug: 19029490
Change-Id: I6222edcab8e394e5fb6adf7a982be446e4505a1e