[roll] Roll fuchsia [audio_core] Fix ramp-on-Pause inconsistency

Currently, upon receiving a Pause call AudioRenderers ramp their
output down to silence over 5 ms, then change their transport
state to paused and restore the original stream gain. Depending
on the timing of the mix cadence, this may cause the final
gain-restoral to occur before ramp-down completes -- or even
before it starts at all.

On top of observed pauses that are less smooth than expected,
we see related test flakes, in audio-renderer-pipeline-test.

This CL inserts additional delay between ramp-down end and
subsequent gain-restoral, which guarantees that the ramp
completion occurs in a different mix period from the subsequent
gain-restoral.

Along with additions to the PauseRampDown case to isolate the
problem (increasing its failure rate to 19%), the fix eliminates
the flakes.

Multiply: audio-renderer-pipeline-test
Original-Fixed: fxb/86405

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/624161
Original-Revision: 519b1cc5b7c861496603dbbad49ddced24c53a8c
GitOrigin-RevId: 12b6aff925fa5544ef9845bd55aebcc0332a5dc3
Change-Id: Ib6f2b8bf50b765d18dc5de24b51b32912e8a33be