KeyringController: Update to BaseControllerV2 #1258

This is part of the effort to consolidate keyring interactions between extension and mobile.
Originally proposed here.

Context

Right after instantiating the EthKeyringController, the extension subscribes to three keyring events: onLock, onUnlock and to memStore observable.
The core KeyringController has onLock, onUnlock and subscribe methods to do that, but subscribe subscribes to the store, not the memStore.

Proposal

We can consider updating core KeyringController to BaseControllerV2, which allows subscription on persistent and non-persistent data.