audio policy: fix concurrent capture policy for virtual sources

Do not consider virtual sources when determining
lastest active capture client. As a virtual source
capturing does not silence other captures, it should not
prevent another client from being selected as the latest active.

Bug: 160820363
Test: Verify capture with Solotester in background and projecting to
DHU.

Change-Id: I2f28026dbecfa72b5d44f52b95538fc13996627b