Work on issue #18640385: Add procstats test mode

You can now do "adb shell dumpsys procstats --start-testing" to
enable high frequency pss sampling.

Also improved the low on RAM mem reporting to separate out RAM
from memtrack, in case the data we are getting from that is bad.

And fixed meminfo --oom to work correctly again.

Change-Id: I7af17eab110a82298bd7b0ce381f8fa5c96c1f6a