Rework SUBM label in LabelId

Use a static method instead of a public LabelId field. LabelId is
mutable, so naming it like a constant is misleading at best. At worst,
a caller could call SUBMIT.set(newValue) and change the value seen by
all callers in the process. (Fortunately no callers were doing this as
far as I could tell.)

While we're in there, rename methods to refer to this as a "legacy"
submit label, as we want to eventually get rid of it in NoteDb.

Change-Id: Id7fbddfaac1d625839c66c73231d4e4ba2550c50

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java

@@ -611,7 +611,7 @@ public class PostReview implements RestModifyView<RevisionResource, ReviewInput>
 
       for (PatchSetApproval a : approvalsUtil.byPatchSetUser(
           ctx.getDb(), ctx.getControl(), psId, user.getAccountId())) {
-        if (a.isSubmit()) {
+        if (a.isLegacySubmit()) {
           continue;
         }