ion: Register with show_mem notification framework

Bugs in Ion clients are semi-common cause of out of memory
errors. Register with the show_mem notification framework
to let Ion dump out data for debugging. To avoid having to
take an excessive amount of locks, keep a running total of
allocated and potentially orphaned memory.

Change-Id: I52c31459d5bf975c67adb5f3fe17033c8e01d91b
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>