Rename postinstall_mount_device to readonly_target_path

When postinstall_mount_device is initially introduced, it's only
intended to be used by postinstall action, hence  the name. Now we plan
to use it for fs verification purpose as well, rename for better
clarity.

Test: th

Change-Id: Iff996f2f513bb44694e39d758a69851793b9a565
diff --git a/common/dynamic_partition_control_interface.h b/common/dynamic_partition_control_interface.h
index d5e1d8d..a5be6e1 100644
--- a/common/dynamic_partition_control_interface.h
+++ b/common/dynamic_partition_control_interface.h
@@ -39,7 +39,7 @@
 
 struct PartitionDevice {
   std::string rw_device_path;
-  std::string mountable_device_path;
+  std::string readonly_device_path;
   bool is_dynamic;
 };