[Bluetooth] fix initialization in IVI and getDefaultAdapter()

bluetooth-frwk doesn't handle correctly the case that the bluetooth is
powered and its framework is off. In that case, we call a hackish method
in order to start bt-service daemon via dbus service activation.

This patch also prevents from being blocked in getDefaultAdapter() method.

BUG=XWALK-2419
I don't set `get_default_adapter_ = false` in `BluetoothInstance::OnStateChanged()`, I do it in `BluetoothInstance::GetDefaultAdapter()` after sending the response to javascript.