Blacklist versions as part of Rollback along with unittests.

This CL adds version blacklisting as part of AU Rollback. A few additional
things:

1) Since this pref must persist across rollback I have introduced a
powerwash_safe_prefs as part of system_state that will persist across
powerwashes.
2) Fixed bug where we needed to read the device policy (which is read during an
update_check before Rollback would work).
3) Some refactoring to move pref constants to constants.
4) Passing keepimg into our powerwash command so we don't wipe the old
partitions.

BUG=chromium:252589 chromium:254217
TEST=Unittests + test on device + using rollback with and without powerwash
checking preserve state.

Change-Id: I991fad944594944425fd9941e10b30a919f2b83b
Reviewed-on: https://gerrit.chromium.org/gerrit/59518
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
diff --git a/real_system_state.h b/real_system_state.h
index 83f3ac1..9ced628 100644
--- a/real_system_state.h
+++ b/real_system_state.h
@@ -52,6 +52,10 @@
     return &prefs_;
   }
 
+  virtual inline PrefsInterface* powerwash_safe_prefs() {
+      return &powerwash_safe_prefs_;
+    }
+
   virtual inline PayloadStateInterface* payload_state() {
     return &payload_state_;
   }
@@ -95,6 +99,9 @@
   // Interface for persisted store.
   Prefs prefs_;
 
+  // Interface for persisted store that persists across powerwashes.
+  Prefs powerwash_safe_prefs_;
+
   // All state pertaining to payload state such as
   // response, URL, backoff states.
   PayloadState payload_state_;