Handle NPE in HFPClientConnectionService

Problem: Telecom manager can return null phone account from a phone account
handle. Hfpclient service tries to retrieve the device from phone account's
address. Since phone account is null, it results in NPE.

Solution: Add a null check.

Bug: 221861375
Test: Added new unit-test to ensure Hfp service handles null data from
telecom manager.
Tag: #stability

Change-Id: I915ada921c40b6d9398ecad1f52f62016977b262