cpufreq: interactive: don't drop speed if recently at higher load

Apply min_sample_time to the last time the current target speed
was originally requested or re-validated as appropriate for the
current load, not to the time since the current speed was
originally set.  Avoids periodic dips in speed during bursty
loads.

Change-Id: I250bda657985de60373f9897cc41f480664d51a1
Signed-off-by: Todd Poynor <toddpoynor@google.com>