audio policy: add support for device combo with speaker safe

combo devices with AUDIO_DEVICE_OUT_SPEAKER_SAFE were not
supported by audio policy manager causing selection
of speaker+headphone when spekaer_safe+headphone would be
the right selection.

Bug: 21537010.
Change-Id: I9865352559c9d32c6754ad3d2b84bddfe2dc8aac