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.cc b/aosp/boot_control_android.cc
index bda65be..3b20fc2 100644
--- a/aosp/boot_control_android.cc
+++ b/aosp/boot_control_android.cc
@@ -30,8 +30,6 @@
 
 using std::string;
 
-using android::dm::DmDeviceState;
-using android::hardware::hidl_string;
 using android::hardware::Return;
 using android::hardware::boot::V1_0::BoolResult;
 using android::hardware::boot::V1_0::CommandResult;
@@ -183,4 +181,12 @@
   return dynamic_control_.get();
 }
 
+std::optional<PartitionDevice> BootControlAndroid::GetPartitionDevice(
+    const std::string& partition_name,
+    uint32_t slot,
+    uint32_t current_slot,
+    bool not_in_payload) const {
+  return dynamic_control_->GetPartitionDevice(
+      partition_name, slot, current_slot, not_in_payload);
+}
 }  // namespace chromeos_update_engine