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/mock_payload_state.h b/mock_payload_state.h
index 3dcf03c..8959ca1 100644
--- a/mock_payload_state.h
+++ b/mock_payload_state.h
@@ -27,6 +27,7 @@
   MOCK_METHOD1(UpdateFailed, void(ErrorCode error));
   MOCK_METHOD0(ShouldBackoffDownload, bool());
   MOCK_METHOD0(UpdateEngineStarted, void());
+  MOCK_METHOD0(Rollback, void());
 
   // Getters.
   MOCK_METHOD0(GetResponseSignature, std::string());
@@ -41,6 +42,7 @@
   MOCK_METHOD1(GetCurrentBytesDownloaded, uint64_t(DownloadSource source));
   MOCK_METHOD1(GetTotalBytesDownloaded, uint64_t(DownloadSource source));
   MOCK_METHOD0(GetNumReboots, uint32_t());
+  MOCK_METHOD0(GetRollbackVersion, std::string());
 };
 
 }  // namespace chromeos_update_engine