cpufreq: interactive: use default sample time 20ms

Lower the default time at which a higher speed is allowed to run
before lowering based on lower CPU load from 80ms to 20ms.  Most
Android devices should trade power for performance here,
although tablets and non-battery-powered devices may want to
override this default.

Change-Id: I1a4f7faeca12793c51d5b92db30a63cca8d4f1be
Signed-off-by: Todd Poynor <toddpoynor@google.com>