Blen is the Bluetooth Low Energy (ble) N-th library, for Android API 21 as minimum.
It's customizable with one standard Java Observer, onto DeviceHost, or with two Observer one for device scanning and the other one for device connection lifecycle.
The only device tested is CC2650 SensorTag.
There is an app, able to scan nearby devices, connect to one of them, list the services, read and (partially) write characteristics.