Activate audio focus when a call enters CONNECTING

Takes audio focus when a call is connecting instead of dialing, and
allows connecting calls to be foreground. This fixes 27356933 by
returning a non-null value from CallsManager#getForegroundCall in
BluetoothPhoneServiceImpl#hangupCall.

Bug: 27356933
Change-Id: I4f7fcc3eae5408cc1beaa77a73419b6072666db6