HFP is exposed via Telecom ConnectionService.

Telecom provides a ConnectionService mechanism where in order to make
calls you only need to provide the right phone account (which can be
queried using a predefined scheme).

This change adds a new middleware called ConnectionService which
provides a translation medium from Bluetooth <-> Telecom. Anyone who
wishes to use HFP HF role can then simply use TelecomManager (and
TelecomManager's InCallService) interfaces.

Bug: b/26757899

Change-Id: I66e47b6ff6330cfd9040a4a6cf4edadac28d63de