floss: Implement UnregisterClient and ClientConnect

This implements IBluetoothGatt's UnregisterClient and ClientConnect
according to the Binder implementation. Also introduces ContextMap
structure to store GATT API client info.

Bug: 193685325
Tag: #floss
Test: Manual - build floss on Linux and test with WIP btclient

Change-Id: Icf20fc6fc09ee283d0133f5a78bac7a5cce395d5