bluetooth: Fix replying to getDefaultAdapter() before it is ready

Before we can reply to that message we need to be sure that we have
all the information about the adapter (its path, address, for example)
or that information can't be obtained (BlueZ isn't running).
bluetooth: Cancel all pending calls on the destructor

We explicitly leak the GCancellable, because the calls are cancelled
asynchronously and they may survive BluetoothContext. We will only
free them on the shutdown callback of the extension.