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