Bug 1072075: don't allow system time changes to affect app usage timings
(cherry picked from commit 83af4279c67b36c96b04ffec04ae428b69eec51d)

reference bug 1069863 in comments

fix broken unit tests
Merge pull request #24855 from davidflanagan/bug1072075

Bug 1072075: don't allow system time changes to affect app usage timings r=marshall