bullhead: Add speaker + bt-sco combo device

* Add path entry for speaker + bt-sco combo device in mixer_path.xml
* Add backend support for such path in audio_platform_info.xml
* Removed composite paths starting with bt-sco- as they are never
  selected by the platform code in the HAL
* Enable in-band ringing in config.xml overlay

Bug: 19171297
Test: Code compilation, testplans/79248
Change-Id: Ib507da5eb0e349821799630ac6e38f5908ff0530
3 files changed