* 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
0b9e581c36