Avoid reading sysprops again.

DynamicPartiitonControlAndroid reads sysprops
again and again. These feature flags can be stored
as a member variable.

Test: pass
Change-Id: Ie885afa502ae7387b76c231d9f54cd550c38c682
diff --git a/dynamic_partition_control_android.h b/dynamic_partition_control_android.h
index 35d8216..d70a2aa 100644
--- a/dynamic_partition_control_android.h
+++ b/dynamic_partition_control_android.h
@@ -112,6 +112,8 @@
                                           const DeltaArchiveManifest& manifest);
 
   std::set<std::string> mapped_devices_;
+  const FeatureFlag dynamic_partitions_;
+  const FeatureFlag virtual_ab_;
   std::unique_ptr<android::snapshot::SnapshotManager> snapshot_;
   bool target_supports_snapshot_ = false;