Only check for powerwash in A/B ota packages

* A-only doesn't have payload_properties.txt
  causing an user-facing error

Change-Id: If749c9a8cf1c3bbdf0300a2da06ec2246fc75484
diff --git a/install/install.cpp b/install/install.cpp
index 83f3cad..044856b 100644
--- a/install/install.cpp
+++ b/install/install.cpp
@@ -551,7 +551,9 @@
   } else {
     LOG(FATAL) << "Invalid status code " << status;
   }
-  PerformPowerwashIfRequired(zip, device);
+  if (package_is_ab) {
+    PerformPowerwashIfRequired(zip, device);
+  }
 
   return INSTALL_SUCCESS;
 }