On Adapter::poweredChanged, check if we need to modify usableAdapter

One of the things we do in usableAdapter is check if the adapter is
powered, if it is not then we consider the adapter not usable.

It happens that an adaptor is added but it is not powered so we discaard
it as an usableAdapter. When that happens we have to make sure that we
are listening to "poweredChanged" and check if usableAdapter should be
changed when that happens (an adapter has become powered, use it in case
we do not have any usableAdapter).

this whole usableAdapter thing is a mess we have to revise for 2.0

CCBUG: 314356