ANDROID: sched: Add vendor hook for util_fits_cpu

Vendor may have the need to implement their own util_fits_cpu
function. This hook is needed until we get the rest of our work upstream
merged to improve definition of migration margin.

Bug: 344826816
Bug: 261704404
Change-Id: I0318f6cadd5c45a37f1acafdded479b25e520e6b
Signed-off-by: Chungkai Mei <chungkai@google.com>
(cherry picked from commit ec3c9a17)
Signed-off-by: Qais Yousef <qyousef@google.com>