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/update_attempter.h b/update_attempter.h
index a9dadf0..a98fcbf 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -147,8 +147,8 @@
   // This is the internal entry point for going through a rollback. This will
   // attempt to run the postinstall on the non-active partition and set it as
   // the partition to boot from. If |powerwash| is True, perform a powerwash
-  // as part of rollback.
-  void Rollback(bool powerwash);
+  // as part of rollback. Returns True on success.
+  bool Rollback(bool powerwash);
 
   // Initiates a reboot if the current state is
   // UPDATED_NEED_REBOOT. Returns true on sucess, false otherwise.