Change definition of warmup period

Previously, warmup was considered done as soon as any write() took
more than 0.5 nominal cycle time.  In practice, this was always the
first write() that turned on power to the output path, and it didn't
accurately account for filling the full kernel buffer queue, or for
buffering in the HAL sample rate conversion.

Now warmup is considered done when a write() _after_ the first write
takes more than 0.5 nominal cycle time.

This will throttle the initial pull rate after coming out of standby.
When combined with another change to throttle the pull rate for
devices with HAL sample rate conversion, it may help reduce some of the
notification glitches.  The only downside is that it will increase the
warmup time a bit.

Bug: 6508317, 6650846, 6607056
Change-Id: I39f324c5195578170a55308e9601d3a1b41db3e0