In many cases we just used "playback speed" as a detailed Javadoc
parameter or return type definition. This doesn't define which scale
the speed is using.
PlaybackParameters as the main point to set the speed already uses a
more precise wording to describe the value as a factor by which playback
will be sped up.
This change replaces other usages of "playback speed" with this wording
whereever we would usually add a unit, keeping "playback speed" for
summary statements etc to reference the general concept that doesn't
usually require a unit.
PiperOrigin-RevId: 350379139
25ed6b127d