ANDROID: scheduler: export task_sched_runtime

For power and performance monitoring, need to known tasks' runtime for
loading estimation.

But now, other modules can't get task_scehd_runtime.

Export task_sched_runtime to let other modules get task_scehd_runtime.

Bug: 195914330
Signed-off-by: Poting Chen <poting.chen@mediatek.com>
Signed-off-by: Cheng Jui Wang <cheng-jui.wang@mediatek.com>
Change-Id: Ida5caf8ed0a32954fc0b0ed950f163c7ca493fef