Add BluetoothDeviceManager (multi-hfp part 1)

Adds BluetoothDeviceManager, a class to manage connected HFP devices for
Telecom. This class separates out part of the current functionality of
BluetoothManager and generalizes it to support multiple connected HFP
devices. See design doc for details.

Test: added unit tests
Change-Id: I03ca6e3827aa80b18e64a9c736097ca8dd4e01b8