Add AutoValue classes for Preferences

AccountState is a cached entity that contains user preferences. Any
cached entity should only hold onto immutable objects and be immutable
itself. This commit addds AutoValue classes for user preferences that
replace the *Info objects in AccountState.

For now, the change is internal to AccountState only and the rest of the
code is not touched. In the future, we might keep *Info classes only on
the API level and use the immutable AutoValue representation in the
inner works of Gerrit. Any such change can be an interative follow up.

We use Optionals for now to express that any field can be unset. The
alternative here is @Nullable. For now, Optional seems to be the better
choice as it makes callers explicitly aware of that fact. This can be
adapted any time.

The alternative to making these classes AutoValues would be either to
defensively copy them when storing and retrieving them from AccountState
or using Protobuf. In the former case, the required boiler plate would
be similar to what is implemented here as we would need copy methods as
well as equals and hashCode. The latter case is very similar to
AutoValues, but we don't pass protos around in Gerrit until now.

This change explicitly resets the user in two test cases as the test
suite holds on to an IdentifiedUser which in turn holds on to an
AccountState. Prior to this commit, we would mutate these values when
updating the preferences. With the new mechanism, that is not the case
anymore (this is the correct state of the world).

Change-Id: I8ab306e8d1d8f5d1d9ce3209aaa600347ed47e5c