TODO: Replace Bluetooth LE Scan Logic with version compatible for Android Version < 5.0

[rgreschner]

The Bluetooth LE Device Scan Logic was replaced with BluetoothLeScanner introduced in Android API Level 21 (see https://developer.android.com/reference/android/bluetooth/le/BluetoothLeScanner.html).

The used implementation also needs to support devices < API Level 21 in the future (for now, focus is on presentation).