sched/fair: Fix util_est UTIL_AVG_UNCHANGED behaviour

The util_est internal UTIL_AVG_UNCHANGED flag which is used to prevent
unnecessary util_est updates uses the LSB of util_est.enqueued. It is
exposed via _task_util_est() (and task_util_est()).

Commit 92a801e5 ("sched/fair: Mask UTIL_AVG_UNCHANGED usages")
mentions that the LSB is lost for util_est resolution but
find_energy_efficient_cpu() checks if task_util_est() returns 0 to
return prev_cpu early.

Because of _task_util_est() returning the actual util est value or'ed w/
UTIL_AVG_UNCHANGED for a dequeued task this can never be true w/
SCHED_FEAT(UTIL_EST, true).

To fix this use the MSB of util_est.enqueued instead and keep the flag
util_est internal, i.e. don't export it via _task_util_est().

MSB of unsigned int util_est.enqueued shouldn't be used for a task
since the maximal possible util_avg value for a task is 1024.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>