Report SwapPss usage if available as part of Pss

Parse "SwapPss:" lines from /proc/pid/smaps if it exist, and store them
in a seperate stat entry.
Report SwapPss if made available by kernel, otherwise we fall back to
legacy Swap.

Fix getTotalSwappablePss documentation.

Change-Id: I361928c0f44c7dc9b959b91c127c916215063866
Signed-off-by: Thierry Strudel <tstrudel@google.com>