audio policy: implement routing policy for USB docking stations

USB docking stations with speaker use device type
AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET.

A docking station is not treated as a regular USB headset in that it
does not follows the same rules as wireless or wired headsets which
requires that media is routed to the most recently connected device.

Bug: 197584191
Test: make
Change-Id: I461661dd42190f513632d1df3a1583f9d82efc1e