Merge "Document libhidltransport removal plan." am: e21d1b6ffc am: cc341c5f29 am: 2224f7ec88

Original change: https://android-review.googlesource.com/c/platform/system/libhidl/+/1356017

Change-Id: Ib057b5d6289e6227bd7055b56eb0ae7ca50c4c10