Keep timestamp in aar.py and copy_ex.py when file is not changed

This removes unnecessary build steps in incremental build.

Also this mitigates crbug.com/860251 a bit.

Bug: 810298, 860251
Change-Id: Ia6aedfea692611a08b1f9a6a2c3963bb69cf32bc
Reviewed-on: https://chromium-review.googlesource.com/1127565
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573145}
2 files changed