Monitor swap

Pull value of VmSwap from /proc/PID/status when capturing
ProcessMemoryState atom.

Before change:
(average pull time nanos) 43355896
(max pull time nanos) 75649278

After change:
(average pull time nanos) 86307073
(max pull time nanos) 151681474

Delta: 2x increase
Pulling frequency is controled via statsd and we trade-off more
expensive reads for more actionable data (helping detect memory leaks).

Bug: 130624561
Test: atest MemoryStatUtilTest
Test: benchmark pulling ProcessMemoryState atom
Test: manually take a statsd report
Change-Id: I1d90563b70b5253b3d31ddab4810db870620c4d4
(cherry picked from commit 65a9b712)