Don't assume that transport unbind == transport invalid

We now expect that the transport service's host process will be
OOM-killed sometimes, but will come back automatically because we
still hold the binding.  So, we no longer drop it as the
expected-to-be-current transport when that happens.

Bug 10728767

Change-Id: I5e756e8942e7c4c3567632f10460ee31b9618d75