Enable access to display/manager socket in surfaceflinger

DisplayManager interface is needed by VrCore. Currently the entry
point for that service is not accessible outside of `graphics` user
group which VrCore is not part of. We need to allow access to that
socket (and subsequently use selinux to fine-tune access rights
in the future).

Bug: 37486608
Test: `lunch sailfish-eng && m -j32` works and device boots
Change-Id: I1d1b545ba85aa6700b409fbbdacc330b8302049b