Error Prone: Enable and fix ImmutableEnumChecker

* Mark some enums using @Immutable from Error Prone (and also import
  that library).
* Suppress one usage of an effectively-immutable JGit class which we
  don't control.
* Fixing ReviewerStateInternal to store a String internally rather than
  a FooterKey. FooterKey is actually not immutable: it has a package-
  private byte array field. Keep returning FooterKey from the interface
  for type safety; given that one method was already constructing a new
  FooterKey dynamically on every invocation, and that this class is
  primarily used for constructing ChangeUpdates, this shouldn't be a
  performance issue.

Change-Id: Ieb7ee10e9be39184cff7e0d1d18ff6b433016b31

java/com/google/gerrit/server/change/ArchiveFormat.java

@@ -35,7 +35,9 @@ public enum ArchiveFormat {
   TXZ("application/x-xz", new TxzFormat()),
   ZIP("application/x-zip", new ZipFormat());
 
+  @SuppressWarnings("ImmutableEnumChecker") // ArchiveCommand.Format is effectively immutable.
   private final ArchiveCommand.Format<?> format;
+
   private final String mimeType;
 
   ArchiveFormat(String mimeType, ArchiveCommand.Format<?> format) {