cpufreq: interactive: Support variable frequency calculations

Use a threshold value up to which we are evaluating the current
speed weighted by current load against each new choice of speed.
Above switch to the original frequency calculation proportional to
load relative to the max possible frequency which allows for more
dynamic and fair changes.

Default threshold is cpuinfo min which guarantees minimal freqs
and energy usage during (offloaded) media playback.

Also, choose the highest frequency below or at target, which
generally saves us one freq level.

Change-Id: Ic807cc89a6d5cfb47f353b0912ced489baae8cbf