android: Measure startup histograms starting from activity creation

This CL starts using activity creation time instead of applicaton creation
time for the Startup.Android.Experimental.Cold.TimeToFirst{ContentfulPaint,NavigationCommit}
histograms.

It's done in order to reduce histograms noise. These histograms are still
experimental, so renaming isn't required.

Bug: 778302
Change-Id: Ic615c14d62566bb31fec651e706ded158abf9d99
Reviewed-on: https://chromium-review.googlesource.com/977968
Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546068}
3 files changed