Keep Android X64 shards on the same machine type as parent.

Change-Id: Iea791a47a137f8836d22a286ce6241dd09c750da
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/513400
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>