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/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index d51241f..bd49639 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -50,7 +50,6 @@
 
 namespace chromeos_update_engine {
 
-using brillo::MessageLoop;
 using std::string;
 using std::vector;
 
@@ -127,7 +126,7 @@
   const InstallPlan::Partition& partition =
       install_plan_.partitions[current_partition_];
 
-  const string mountable_device = partition.target_path;
+  const string mountable_device = partition.postinstall_mount_device;
   if (mountable_device.empty()) {
     LOG(ERROR) << "Cannot make mountable device from " << partition.target_path;
     return CompletePostinstall(ErrorCode::kPostinstallRunnerError);