[Bluetooth] Fix not passing a SuccessCallback to stopDiscovery()

When should check if successCallback passed to stopDiscover() exists
(if it exists, its type is already correct) before calling it.
[Bluetooth] Fix sending wrong reply to BluetoothAdapter.stopDiscovery()

The reply id of the call should be cleared after it is handled.
Merge pull request #227 from vcgomes/bluetooth-fix-stop-discovery

[Bluetooth] fix BluetoothAdapter.stopDiscovery()