cpufreq: interactive: Simplify frequency calculations

Previously, complicated calculations led to oscillating cpu
frequencies that ruined user experience even on high-end
devices. Revert to the classic calculations based on a
percentage of current max frequency.  This way, we're using
freqs more evenly and much more predictable.

Change-Id: Id7898618607f0e26b272e3c74932a92380e3db7a