update_engine: Rename policy values of DeviceRollbackToTargetVersion

Rename the following values for better understanding and
consistency:
- RollbackWithFullPowerwash to RollbackAndPowerwash
- RollbackWithRestoreOnly to RollbackOnlyIfRestorePossible

BUG=chromium:878736
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: Ib4129d2c297697e850d3027d35599fc37e104b0e
Reviewed-on: https://chromium-review.googlesource.com/1198533
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/update_manager/rollback_prefs.h b/update_manager/rollback_prefs.h
index 07e1834..11d09d6 100644
--- a/update_manager/rollback_prefs.h
+++ b/update_manager/rollback_prefs.h
@@ -29,9 +29,9 @@
 enum class RollbackToTargetVersion {
   kUnspecified = 0,
   kDisabled = 1,
-  kRollbackWithFullPowerwash = 2,
+  kRollbackAndPowerwash = 2,
   kRollbackAndRestoreIfPossible = 3,
-  kRollbackWithRestoreOnly = 4,
+  kRollbackOnlyIfRestorePossible = 4,
   // This value must be the last entry.
   kMaxValue = 5
 };