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/payload_consumer/install_plan_unittest.cc b/payload_consumer/install_plan_unittest.cc
index 2ca8d81..7779494 100644
--- a/payload_consumer/install_plan_unittest.cc
+++ b/payload_consumer/install_plan_unittest.cc
@@ -38,6 +38,7 @@
           .source_path = "foo-source-path",
           .source_hash = {0xb1, 0xb2},
           .target_path = "foo-target-path",
+          .readonly_target_path = "mountable-device",
           .target_hash = {0xb3, 0xb4},
           .postinstall_path = "foo-path",
           .filesystem_type = "foo-type",
@@ -66,6 +67,7 @@
   target_hash: B3B4
   run_postinstall: false
   postinstall_path: foo-path
+  readonly_target_path: mountable-device
   filesystem_type: foo-type
 Payload: 0
   urls: (url1,url2)