daemon: Introduce low-energy mode

Once activated the low-energy mode prolongs the beaconing period to 60 seconds.
Each beacon has an period appended to signal the current period to the receivers.
daemon: Add setLeMode() to NativeDaemon
android-daemon: Introduce LE mode when screen is off

If the screen is off the daemon switches to LE mode which prolongs discovery periods
to one minute.
android-daemon: Remove network-state listener from AndroidManifest

The listener for network-states is removed to prevent unnecessary launches of 
the service while these are turned off. Instead the service instantiates a BroadcastReceiver
as soon as the daemon is in online state.
Merge pull request #57 from morgenroth/Add-LE-mode

Introduce Low-Energy state