agl-service-bluetooth: Fix race condition between powering of adapter and starting of discovery

From adapter-api.txt:
" When changing the Powered property the new state of
this property will be updated via a PropertiesChanged
signal.
"

This has been noted in dbus-monitor and error codes of afb calls that enabling discovery can be requested before the adapter is powered on. Proper solution is to send adapter_changes event from PropertiesChanges dbus results.