Invalidate stream when unregistering audio policy mix

Invalidate the stream if the audio policy mix on the client is removed,
so that the client can reconnect to another output stream.

Bug: 167605686
Test: Unregister audio policy by killing the app. Audio resumes.
Change-Id: I9a00311142613df8c019206c9b7c873ec8f42b88
2 files changed