cpufreq: interactive: Separate speed target revalidate time and initial set time

Allow speed drop after min_sample_time elapses from last time
the current speed was last re-validated as appropriate for
current load / input boost.

Allow speed bump after min_sample_time (or above_hispeed_delay)
elapses from the time the current speed was originally set.

Change-Id: Ic25687a7a53d25e6544c30c47d7ab6f27a47bee8
Signed-off-by: Todd Poynor <toddpoynor@google.com>