Correct /sys/kernel/tracing permissions

In Android kernels >4.4 we will see an empty /sys/kernel/tracing
directory which is notionally where you should mount tracefs if you
don't want to mount debugfs. As we move towards not mounting debugfs,
ensure that the non-legacy location also has adequate permissions to be
read by tracing tools.

Note that this change will be OK even if the board init.rc doesn't mount
tracefs here, because sysfs will always create this directory.

Bug: 148436518
Change-Id: I674587d0f08effdb8471a82e3b1ceec3af8588de