ANDROID: kernel: sched: Export reweight_task

Export reweight_task for vendor usage when they are trying to manipulate
task prio. After the prio changed, it will need to update its load
weight to take effect. Therefore, this function needs to be called
from vendor kernel module. It could be used with
trace_android_rvh_set_user_nice and trace_android_rvh_setscheduler.

Bug: 245675204
Change-Id: I0033518bf1cbd0a8129795743b95340f439d5fe8
Signed-off-by: Rick Yiu <rickyiu@google.com>