r_submix: Use intermediate pipe in non-blocking mode

Switch the pipe into non-blocking mode to prevent stalling
when attempting to close it.

Simulate circular buffer behavior by flushing old data if
there is no more space in the pipe.

This removes the need for "exiting=1" parameter.

Bug: 73175392
Test: r_submix_tests
Change-Id: Iff89980af71112892ff262030e471ae736b1f62a
2 files changed