[Bluetooth] Only reply Adapter.getDefaultAdapter() when adapter is ready

We call ready, when the adapter is in a known state, whether it is ready and
we were able to get all its properties, or we couldn't obtain it (bluetoothd is
not running or the adapter path is not available).
Merge pull request #229 from vcgomes/fix-set-powered-bug-v2

[Bluetooth] Only reply Adapter.getDefaultAdapter() when adapter is ready