Convert PatchSetApproval to AutoValue

The public API of PatchSetApproval remains the same with respect to
nullability: getTag is a non-nullable Optional, and getRealAccountId
falls back to getAccountId if setRealAccountId was never called. In the
old implementation, these fields were nullable internally, due to
ReviewDb constraints; AutoValue has no such constraints, so they are
plain non-nullable fields. Instead, populating the default values is
done via logic in the builder.

The no-arg defensive copy method is no longer required, since
PatchSetApproval is immutable. copyWithPatchSet is still useful as a
shortcut, because swapping out just the PatchSet.Id nested in the
PatchSetApproval.Key is annoying.

Leave method names the same so that substantive code changes are easier
to review.

Change-Id: I342f87e9a7cd209afa6e4a6d0158a1f3a5d11360

java/com/google/gerrit/server/notedb/ChangeNotes.java

@@ -62,7 +62,6 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
-import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.function.Predicate;
@@ -338,12 +337,7 @@ public class ChangeNotes extends AbstractChangeNotes<ChangeNotes> {
 
   public ImmutableListMultimap<PatchSet.Id, PatchSetApproval> getApprovals() {
     if (approvals == null) {
-      ImmutableListMultimap.Builder<PatchSet.Id, PatchSetApproval> b =
-          ImmutableListMultimap.builder();
-      for (Map.Entry<PatchSet.Id, PatchSetApproval> e : state.approvals()) {
-        b.put(e.getKey(), e.getValue().copy());
-      }
-      approvals = b.build();
+      approvals = ImmutableListMultimap.copyOf(state.approvals());
     }
     return approvals;
   }