[AW][Dev-UI] log duration for user session

No change to logic. This adds logs to capture the duration of the user's
session with each tool. This captures each tool in a different
histogram, measuring from onResume until onPause (so we intentionally
don't include time when the app is in the background).

Fixed: 1084177
Test: Add a log statement and verify it fires as expected
Change-Id: I9933a97c5c99d123440ed4e46b94bc056964ada3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211146
Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771143}