meminfo: Remove unnecessary working set stats

The Vma and ProcMemInfo objects do not need separate stats
objects for storing working set. The Vma either has working set
or memory usage information and never both.

Bug: 111694435
Test: libmeminfo_test 1

Change-Id: I2df05f7e750bbba4325474633e705d6d68dd2ccb
Merged-In: I2df05f7e750bbba4325474633e705d6d68dd2ccb
Signed-off-by: Sandeep Patil <sspatil@google.com>