Add reserved disk GID to critical component.

We recently created a new GID that can be granted to critical system
processes, so that the system is usable enough for the user to free
up disk space used by abusive apps.

Test: builds, boots
Bug: 62024591
Change-Id: Ia5af7535cc05a214f8720ac08c594c6db888597a
diff --git a/rootdir/init.zygote64.rc b/rootdir/init.zygote64.rc
index 05ec16f..6fc810b 100644
--- a/rootdir/init.zygote64.rc
+++ b/rootdir/init.zygote64.rc
@@ -2,7 +2,7 @@
     class main
     priority -20
     user root
-    group root readproc
+    group root readproc reserved_disk
     socket zygote stream 660 root system
     onrestart write /sys/android_power/request_state wake
     onrestart write /sys/power/state on