meminfo: Add Smaps(), showmap and friends.

Needed by showmap and also android_s_Debug to classify each allocation
into multiple heaps.

The APIs added are:
ForEachVmaFromFile - Global API to parse a file expected to be in the
same format as /proc/<pid>/smaps and make a callback for each VMA found.
ProcMemInfo::ForEachVma - Same as 'ForEachVmaFromFile' but for a
ProcMemInfo object corresponding to a process(pid).
ProcMemInfo::Smaps - Wrapper to ProcMemInfo::ForEachVma, except the
function collects 'struct Vma' in a member vector and returns the
reference for the same.

Added showmap2 using the APIs and the corresponding tests the same time.

Bug: 111694435
Test: showmap_test.sh
Test: libmeminfo_test 1

Change-Id: I3065809cf94ecf3da88529809701035c47a8ce34
Signed-off-by: Sandeep Patil <sspatil@google.com>