meminfo: Add support to track working set with idle page tracking.

Also adds a tool to make use of this if it exists call 'wsstop'

Bug: 111694435
Test: wsstop -d 3 -n 100 1

Change-Id: I50415f0bdc09c09b5b414cf0e4fff8f2907c5823
Signed-off-by: Sandeep Patil <sspatil@google.com>