Rename postinstall_mount_device to readonly_target_path am: a9b5d8c8ee am: 6180c7600f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1696827

Change-Id: I9ceacfa5bea6c6d2f3d11798b2621ce6ea3853e3