procrank: do not keep process maps around

procrank's usage in its output shows 20x increase from
last year. This is because it is keeping the process
maps around until termination. Fix that by getting rid
of ProcMemInfo objects when done parsing /proc/<pid>/maps,pagemap.

Note that the total allocations do not change and have not necessarily
regressed from Pie.

Bug: 130672819
Test: adb shell procrank | grep 'procrank\|cmdline'

Change-Id: Ib7bf960ed1d053347fcfc0c8aee9019607a1eb01
Merged-In: Ib7bf960ed1d053347fcfc0c8aee9019607a1eb01
Signed-off-by: Sandeep Patil <sspatil@google.com>