DynamicPartitionsControl: Add Virtual A/B feature flag.

Test: unittest
Bug: 138816109
Change-Id: I7ae65ba0bf36a6ca5085bc4ec2c46245288b4703
diff --git a/dynamic_partition_control_android.h b/dynamic_partition_control_android.h
index 062a2d1..d743e6e 100644
--- a/dynamic_partition_control_android.h
+++ b/dynamic_partition_control_android.h
@@ -30,6 +30,7 @@
   DynamicPartitionControlAndroid() = default;
   ~DynamicPartitionControlAndroid();
   FeatureFlag GetDynamicPartitionsFeatureFlag() override;
+  FeatureFlag GetVirtualAbFeatureFlag() override;
   bool MapPartitionOnDeviceMapper(const std::string& super_device,
                                   const std::string& target_partition_name,
                                   uint32_t slot,