audiopolicy: treat HDMI as part of last connected media device

Recognize HDMI as part of last connected media device.
So, if HDMI is connected last then media should be played
on HDMI.

Test: connect USB Headset then connect HDMI, check playback
Bug: 304884506

Change-Id: Ifa24b8d840e6201588bb4267672976f3918eb273