Use proper lock when accessing the generation registry

We use 'this' for synchronization in NameValueCache but some code
that accesses the generation registry uses 'this' in a different
context ending up syncing on the wrong instance. This is why
sync on this is just a bad idea.

bug:29956424

Change-Id: Ide2d4f07a5f40cb3f0e8f50e4c8de216d15a31ee