Fix permission of zram writeback and idle file

The default permission of "/sys/block/zram0/idle" and
"/sys/block/zram0/writeback" are "0200".

Adding read permission to the files does not make sense because reading
those files fail as EIO error.

We should keep permission as minimum as possible.

Bug: 117682284
Bug: 375432468
Test: ls -la /sys/block/zram0

Change-Id: I11ed5d9eee257002f4698edcd81de39d2c317ea1