Update inactivity tracker so that the pref is always stored on stop.

This also changes the behavior of onStart - it previously cleared the
timestamp too soon for other code to read the pref. Now the timestamp
is stored in memory in the ChromeInactivityTracker even after the pref
is cleared.

Bug: 951874

Change-Id: Ic2c128e899749aac055e3fd2487f6024fb1fbdba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565278
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652193}
6 files changed