LE: Add notification sent and congestion callbacks (1/4)

LE: Add notification sent and congestion callbacks (1/4)

This change introduces two new callbacks for applications to better
handle LE notification flow control and transport congestion. The
notification callback is invoked when the remote platform confirms an
indication or when a local notification has been passed to the
controller. No new notifications should be sent until a callback is
received.

Congestion callbacks are triggered when a GATT operation cannot be sent
to the local Bluetooth controller. Repeatedly calling
writeCharacteristic() for example will eventually trigger a congestion
callback. Applications cannot send additional data until a further
callback is received, indicating that the congestion has cleared up.

Change-Id: I613cbf968770921f853a407888bb8308849039f3