bfq: set background cgroup weight to 10

For better responsiveness of foreground task's io in BFQ I/O scheduler,
set background task's weight from default 100 to 10.

Test: cat /dev/blkio/background/blkio.bfq.weight
Bug: 171739280
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: I6a4ea2d46c4426d3e34c7ea71a4653e0ce3ed0df