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

lib/errorprone/BUILD

@@ -0,0 +1,6 @@
+java_library(
+    name = "annotations",
+    data = ["//lib:LICENSE-Apache2.0"],
+    visibility = ["//visibility:public"],
+    exports = ["@error-prone-annotations//jar"],
+)