audio policy: clarify use of mName field for audio devices

The audio port field mName was used for two different purposes
which caused a problem when exposing this information at the SDK API.

Create a new "mTag" field storing the audio_policy.conf device tag
if needed.
Field "mName" is now used to store any additional name associated to
a given device when available (e.g Manufacturer and model name for a USB device).

Change-Id: I17fa872e4a3a2e1b7cbb807c6f72e095a8a2c9c5