service: Introduce bluetooth::LowEnergyClient

Added the LowEnergyClient class which manages a per-application BLE
API client instance. This currently only supports getting registered
with the stack and obtaining a client_if handle but will expose APIs
in the future for various BLE operations.

Bug: 23793954
Change-Id: I423f6d12321877826525e5e52136682fa9d5a441
12 files changed