Record memcg_device type for compat.

Commit: 86cb5215 gave /dev/memcg a
new label, but also explicitly prohibited access to vendor domains.
Add the type to the 'new types' and don't map it to any other type
for backwards compatibility.

Bug: 62573845
Test: None. Prebuilt change only.
Change-Id: I8902716830b162ead69834544ace9e02a94c65b4

prebuilts/api/26.0/26.0.ignore.cil

@@ -8,6 +8,7 @@
     e2fs_exec
     hal_cas_hwservice
     kmsg_debug_device
+    memcg_device
     netd_stable_secret_prop
     sysfs_fs_ext4_features
     timezone_service