libmeminfo: Add SmapsRollup

This adds the tests and SmapsRollup() parsing function in
ProcMemInfo. Adds tests to check the return value as well as
the correctness.

Bug: 111694435
Test: libmeminfo_test 1 --gtest_filter=TestProcMemInfo.*
Test: libmeminfo_benchmark --benchmark_filter=BM_SmapsRollup_
Result:
----------------------------------------------------------
Benchmark                   Time           CPU Iterations
----------------------------------------------------------
BM_SmapsRollup_old       4751 ns       4730 ns     149458
BM_SmapsRollup_new       4858 ns       4837 ns     144636
----------------------------------------------------------

Change-Id: Ia051fe53a7622e3091502ff7166efafae35e7935
Signed-off-by: Sandeep Patil <sspatil@google.com>