meminfo: Add API to read pagemap for a vma within a process.

.. and make sure we have some helper methods exposed to the clients
to do some basic checks for the pagemap values. For example, to check
if the page is present or swapped.

Bug: 111694435
Test: libmeminfo_test 1 --gtest_filter=ValidateProcMemInfo.TestPageMap
Test: libmeminfo_test 1

Change-Id: Ic6ae91f4214b42346f3d0b54164a43ac79d5ade1
Signed-off-by: Sandeep Patil <sspatil@google.com>