Creating background services for Bluetooth low energy on Android
This functionality has all been moved to the open source Android Beacon Library which will:
wake up/launch your app when iBeacons matching a desired pattern are detected
perform beacon scanning in the background even if the user has not launched your app yet
reduce the scan rate automatically in the background to 30 seconds every five minutes to save battery. (Timing configurable.)
Code examples are show here
If your BLE device is not a beacon you could still use this library to accomplish this by having your sensor also transmit as a beacon then after it is detected connect to the main service.