Do not map dynamic partitions on VABC devices

With VABC, we no longer need to map all partitions before
reading/writing, so don't try to map them.
1. modify GetPartitionDevice to return empty path for target partitions
on VABC
2. Add a separate GetMountableTargetDevice for obtaining a mountable
device path, specifically for postinstall

Test: treehugger
Change-Id: Ib1f608914fc49c677ce7389140ca79b028171191
diff --git a/aosp/boot_control_android.h b/aosp/boot_control_android.h
index e288723..926023a 100644
--- a/aosp/boot_control_android.h
+++ b/aosp/boot_control_android.h
@@ -44,6 +44,11 @@
   // BootControlInterface overrides.
   unsigned int GetNumSlots() const override;
   BootControlInterface::Slot GetCurrentSlot() const override;
+  std::optional<PartitionDevice> GetPartitionDevice(
+      const std::string& partition_name,
+      uint32_t slot,
+      uint32_t current_slot,
+      bool not_in_payload = false) const override;
   bool GetPartitionDevice(const std::string& partition_name,
                           BootControlInterface::Slot slot,
                           bool not_in_payload,