Floss: Implement SendNotification for GATT Server

This implements IBluetoothGatt's SendNotification as well as
corresponding IBluetoothGattServerCallback OnNotificationSent.
This CL also handles congestion.

Bug: 193685149
Tag: #floss
Test: emerge-dedede floss and WIP btclient
Change-Id: Ifa5eec5493b87b688c679989a99341c3e6869a45