floss: Implement BluetoothGatt::RegisterClient

This lays out the basic shape of the implementation of BluetoothGatt
interface. It contains an (incomplete) example of a method,
RegisterClient. The complete implementations of all methods will follow.

Bug: 193685325
Tag: #floss
Test: Build on Linux and AOSP

Change-Id: Icf7ab4c1ba86a3a0e8fdec55984a90e557fa9e64