Initial super-primitive process tracker.

The goal of this is to keep track of what app processes
are doing, to determine who is being abusive, when the system
is getting into memory constrained situations, and help the
user determine how to resolve this.

Right now it doesn't really do any of that, just keeps track
of how long every process has been running since boot.

Also update the activity manager to use "cached" as the terminology
for what it used to interchangeably call hidden and background
processes, and switch ProcessMap over to using ArrayMap.

Change-Id: I270b0006aab1f38e17b7d9b65728679173c343f2