Audio V4: Use string to identify audio Device

The name of an audio device is an opaque string that the
framework does not interpret (there are exceptions, see documentation)
and only uses as an ID to link
the audio_policy_configuration.xml and the audio device factory.

Previously it was an enum which meant that the vendors
could not implement the interface more than a set number of time
and that generic meaningless (secondary, auxiliary) names had
to be introduced.

Bug: 38184704
Test: compile
Change-Id: I9f3ac1928de8116974257951e016108c5820eded
Merged-In: I9f3ac1928de8116974257951e016108c5820eded
Cherry-picked from master
Signed-off-by: Kevin Rocard <krocard@google.com>