libmeminfo: Add ReadSysMemInfo variants

The new variant is primarily used in framework. See: go/ag/5780400
for usage. Also add tests, benchmarks and fix several issues found in SysMemInfo
class.

New benchmark results are:
--------------------------------------------------------------
Benchmark                       Time           CPU Iterations
--------------------------------------------------------------
BM_ReadMemInfo_old           7726 ns       7696 ns      90201
BM_ReadMemInfo_new           7554 ns       7525 ns      90358
BM_ZramTotal_old             6446 ns       6406 ns     108361
BM_ZramTotal_new             6529 ns       6488 ns     106545
BM_MemInfoWithZram_old      14485 ns      14412 ns      48492
BM_MemInfoWithZram_new      20572 ns      20459 ns      33438
--------------------------------------------------------------

The reason for BM_MemInfoWithZram_new shows worse numbers is because
the new API also tries to find more than 1 zram device (if it exists).
The old implementation hard coded everything to "/sys/block/zram0/"

Test: libmeminfo_test 1
Bug: 114325007
Bug: 111694435

Change-Id: I246d9e9a54986ee9b2542d1eaac79ecf7310b23a
Signed-off-by: Sandeep Patil <sspatil@google.com>