service: Add hal::BluetoothGattInterface

Added hal::BluetoothGattInterface which is a C++ wrapper around the
HAL Bluetooth GATT interfaces. This follows the current pattern of hal/*
wrappers and is meant to be fakeable.

Bug: 23395353
Change-Id: Ib11afba9fe2f4632591d5db384edf32ca788bf2c
9 files changed