libmeminfo: Add support to read zram memory consumption

Bug: 114325007
Bug: 111694435
Test: libmeminfo_test 1 --gtest_filter=SysMemInfoParse.TestZramTotal
Benchmark: libmeminfo_benchmark --benchmark_filter=BM_.*ZramTotal
Benchmark Result on Blueline:
-----------------------------------------------------------
Benchmark                    Time           CPU Iterations
-----------------------------------------------------------
BM_OldReadZramTotal       3857 ns       3839 ns     134096
BM_NewReadZramTotal       4461 ns       4440 ns     157341

Change-Id: I5220fa17b101981ef859179960fe78fe68e84852
Signed-off-by: Sandeep Patil <sspatil@google.com>