Keep the sched policy for a thread of low priority

If a thread priority is >= ANDROID_PRIORITY_BACKGROUND, we would like
it keep in background sched policy. However, currently if its main
process is switching among groups such as top-app/fg/bg, the background
policy of that thread will be gone. This patch is to address the problem.

Bug: 139521784
Test: sched policy of thread kept as expected
Change-Id: I19eec263d0ad8f0f2ea770eeeea451e168e0ea83