Fix stack memory accounting for dumpsys

The bug was introduced in refactor done for libmeminfo and stack
memory was getting accounted as native heap as a result.

Test: 'dumpsys meminfo' show non-zero Stack memory in 'Total PSS by
      category
Bug: 132664087

Change-Id: Ifb8418bd9d2e65abc09aa4075d85fd5afc56b57a
Signed-off-by: Sandeep Patil <sspatil@google.com>