Prefer new zram ABI

Starting with Linux 4.1, reading from /dev/block/zram*/mem_used_total
logs this warning:

"Attribute mem_used_total (and others) will be removed. See zram
documentation."

mem_used_total and several related counters are now consolidated under a
new node mm_stat.  Prefer that when it's available.

Bug: 25951511

Change-Id: I0862c3cd1af5d003158aca6be42db25e4bebe7c7
Signed-off-by: Greg Hackmann <ghackmann@google.com>