Revert "Make foreground and background I/O priority different"

It has been reported that CL "Make foreground and background I/O
priority different" causes Android devices equipped with eMMC storage
to reboot if there is sufficient I/O activity. Hence revert commit
7f3fe0aa.

Bug: 186902601
Bug: 342034992
Change-Id: I4fa9c782eb0e801b410cb358e20fe41acebe1094
Signed-off-by: Bart Van Assche <bvanassche@google.com>