Audio V4: Add its own function to open the primary device

The primary device has its own type and a fixed name and
is not optional.

By having its own getter, the client does not have to downcast it.

Bug: 38184704
Test: compile
Change-Id: I7b22eb0e12bddabfe966a20fbeab37d974306b12
Merged-In: I7b22eb0e12bddabfe966a20fbeab37d974306b12
Cherry-picked from master
Signed-off-by: Kevin Rocard <krocard@google.com>