Add a utility function for iterating through all valid process
and thread of each process.
Use it in different parser where we are traversing through the task
list Ex : memusages.py, ion_buffer_parse.py

Change-Id: Ided26acc67e7c5dc2ba7d4c2db0935f0c9fa24f1
Signed-off-by: Ankur Bansal <ankban@codeaurora.org>