audioflinger: refine latency latency calculation.

There is an audio pipe between the normal mixer output and the fast
mixer to cope for scheduling delays and buffer size difference.
This pipe depth was not taken into account in latency calculation.

Adding the pipe contribution to the latency significantly improves A/V sync.

Bug 6520569.

Change-Id: I5584908e8aa8a02170eb38b22b4370eea800a235