DynamicPartitionsControl: Add Virtual A/B feature flag.

Test: unittest
Bug: 138816109
Change-Id: I7ae65ba0bf36a6ca5085bc4ec2c46245288b4703
diff --git a/dynamic_partition_control_interface.h b/dynamic_partition_control_interface.h
index b3ce4ea..9c7b8d0 100644
--- a/dynamic_partition_control_interface.h
+++ b/dynamic_partition_control_interface.h
@@ -50,6 +50,9 @@
   //        NONE iff dynamic partitions is disabled on this device.
   virtual FeatureFlag GetDynamicPartitionsFeatureFlag() = 0;
 
+  // Return the feature flags of Virtual A/B on this device.
+  virtual FeatureFlag GetVirtualAbFeatureFlag() = 0;
+
   // Map logical partition on device-mapper.
   // |super_device| is the device path of the physical partition ("super").
   // |target_partition_name| is the identifier used in metadata; for example,