sched/fair: fix implementation of is_min_capacity_cpu()

With CONFIG_SCHED_WALT disabled, is_min_capacity_cpu() is defined to
always return true, which breaks the intended behavior of
task_fits_max(). Revise is_min_capacity_cpu() to return correct
results.

An earlier version of this patch failed to handle the case when
min_cap_orig_cpu == -1 while sched domains are being updated due to
hotplug. Add a check for this case.

Test: trace shows increased top-app placement on medium cores
Bug: 117499098
Bug: 128477368
Bug: 130756111
Change-Id: Ia2b41aa7c57f071c997bcd0e9cdfd0808f6a2bf9
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>