libmeminfo: defer maps reading only when required for procmeminfo

This restores the original behavior. The main reason it should be this
way is to make the class generic for all things memory under
/proc/<pid>/. For example, with the current behavior, a program that
only needs to read /proc/<pid>/smaps_rollup will end up wasting time and
memory by parsing /proc/<pid>/maps when the object is being constructed.
Same goes for a program that only wants to reset the working set.

The 'ProcMemInfo' object still retains the property that it can only be
used once to read maps and the object must be destroyed + recreated to
update the stats.

Bug: 114325007
Bug: 111694435
Test: libmeminfo_test 1
Test:
 # adb push /google/data/ro/users/ss/sspatil/test-memutils.sh /data/local/tmp/
 # adb push procmem2 /data/local/tmp && adb push procrank2 /data/local/tmp
 # adb root && adb shell
 $ cd /data/local/tmp/
 $ chmod +x test-memutils.sh
 $ ./test-memutils.sh 2>&1 | tee test.log

Change-Id: I856d3b78a0088cff02cbd010b29ffbe0e35f5ee2
Signed-off-by: Sandeep Patil <sspatil@google.com>