Merge "Mark stable and document dalvik.system.CloseGuard" am: 97ea3c03e2 am: 6c2d38dffa am: 08f03e1101

Original change: https://android-review.googlesource.com/c/platform/libcore/+/1660203

Change-Id: If037783b15666c11e12c07c9f79e2a4e3958c28d