getPrimary APIs now examine all non-duplicating output threads

and return the value corresponding to the thread with minimum frame count,
with a preference for a thread with fast mixer if there are multiple
output threads with the same minimum frame count.
This is in keeping with the expectation by apps that the getPrimary APIs
will return a value for the lowest available latency path.

Bug: 29164107
Change-Id: I8ae9bcad2d48185d7717ccf96085834c0ce00e37