Floss: Implement Register/Unregister Server

This implements IBluetoothGatt's RegisterServer and
UnregisterServer as well as IBluetoothGattServerCallback
OnServerRegistered. Also adds these to btclient.

Bug: 193685149
Tag: #floss
Test: emerge-dedede floss and btclient
Change-Id: I54962d453d25716a29029d3ba7f27faab680a746