read battery level while we setup listener

[mdmohsin7]

The listener uses lastValueStream in ble_transport, but the lastValueStream in flutter_blue_plus only emits values when the characteristic changes or when new notifications arrive. It does not emit the current/initial value immediately upon subscription. So we retrieve the battery level while setting up the listener

[beastoin]

man @mdmohsin7 change the logic at the place called performGetBleBatteryLevelListener. keep the omi connection clean and single responsibility in every function.