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/boxed_value.cc b/update_manager/boxed_value.cc
index c987110..9e2971d 100644
--- a/update_manager/boxed_value.cc
+++ b/update_manager/boxed_value.cc
@@ -144,12 +144,12 @@
       return "Unspecified";
     case RollbackToTargetVersion::kDisabled:
       return "Disabled";
-    case RollbackToTargetVersion::kRollbackWithFullPowerwash:
-      return "Rollback with full powerwash";
+    case RollbackToTargetVersion::kRollbackAndPowerwash:
+      return "Rollback and powerwash";
     case RollbackToTargetVersion::kRollbackAndRestoreIfPossible:
       return "Rollback and restore if possible";
-    case RollbackToTargetVersion::kRollbackWithRestoreOnly:
-      return "Rollback with restore only";
+    case RollbackToTargetVersion::kRollbackOnlyIfRestorePossible:
+      return "Rollback only if restore is possible";
     case RollbackToTargetVersion::kMaxValue:
       NOTREACHED();
       return "Max value";