Fixing issue with old tasks being visible in Overview

- This CL does two things, firstly, it ensures that all first & last
  active times are monotonically increasing and independent of the
  current system time.  This allows us to better keep track of which
  tasks are historical and should be hidden, and which are not.
  Secondly, this CL moves the tracking of the last visible active time
  into the system (per user) where it can be adjusted along with the
  task active times when they are loaded.
- Following this CL, all active times in the future will be adjusted on
  boot such that old tasks are made relative to the current boot time.
  It’s not important exactly what time they are, only that they are
  adjusted along with the last visible task active time so that we
  always keep track of what is visible.

Bug: 28908500
Change-Id: I4f789df3a6bd825517cf3a70e26fb60deff89d06